redirect form based on option selection

How do I redirect the user based on two options on drop down menu's?

i have read a few similar questions but couldnt relate the advice as the form code I am using is very long and complicated. I have taken out a lot of the code so I hope i'm not missing anything out thats important.

I am trying to redirect the user based on if they pick 1)Audition Only 2)Workshop Only or 3)Workshop + Audition

As well as if they choose to attend

1) London 2) Birmingham 3) Manchester

So that someone doing an Audition in London would get directed to a different place than someone doing an Audition in Manchester.

If you can I would appreciate the help!

<form name="signupForm" id="signupForm" target="_top" method="post" action="http://ift.tt/1Eh6ET8" onsubmit="return checkMandatory();"> 
<table><tbody>
<tr>
<td> 
<ul id="teamFields"> 

</li> 
<li class="formRow userFieldName" id="field_User8" ><label class="fieldName" id="labelUser8" for="signup_User8">Would you like to attend the workshop and/or audition?<span class="reqMarker">*</span></label> <select name="User8" id="signup_User8"><option value="">Choose</option> <option value="Workshop Only" >Workshop Only</option><option value="Workshop + Audition" >Workshop + Audition</option><option value="Audition Only" >Audition Only</option></select>
</li> 
<li class="formRow userFieldName" id="field_User5" style="display: none;"><label class="fieldName" id="labelUser5" for="signup_User5">Please select the closest city to where you live from the list</label> <select name="User5" id="signup_User5"><option value="">Choose</option> <option value="London" >London</option><option value="Birmingham" >Birmingham</option><option value="Manchester" >Manchester</option></select>
</li> 
<li class="formRow userFieldName" id="field_User21" style="display: none;"><label class="fieldName" id="labelUser21" for="signup_User21">Please select the city closest to where you live</label> <select name="User21" id="signup_User21"><option value="">Choose</option> <option value="London" >London</option><option value="Birmingham" >Birmingham</option><option value="Manchester" >Manchester</option></select>
</li> 
<li class="formRow userFieldName" id="field_User20" style="display: none;"><label class="fieldName" id="labelUser20" for="signup_User20">Please select the closest city to where you live</label> <select name="User20" id="signup_User20"><option value="">Choose</option> <option value="Manchester" >Manchester</option><option value="Birmingham" >Birmingham</option><option value="London" >London</option></select>

</li> 
</ul>
<ul id="teamSelection"> 
<li class="formRow joinRow"><span id="confirmText"> </span>
</li> 
<input type="hidden" name="AutomaticTeamIDs" value="4" /> 
<input type="hidden" name="AutomaticTeamIDs" value="3" /> 
<input type="hidden" name="AutomaticTeamIDs" value="1" /> 
</ul> 
</td> 
</tr> </tbody>
</table> 
<div id="reqInfo">* required information
</div> 
<input id="formSubmit" type="submit" name="formSubmit" style="font-weight: bold; font-size: 16px; color: #000000; border-style: solid; border-width: 8px !important; border-color: #d3d3d3 !important; border-radius: 0px !important; padding: 16px !important; background: #a9a9a9 !important; box-shadow: none !important;" value="Sign Me Up" /> 
<input type="hidden" name="Referrer" value="" /> 
<input type="hidden" name="qstring" value="9ec6ce8b4e17b30b7c2a75edc1c1a684716721de718b41870cfebdceacea9f01" /> 
<input type="hidden" name="formSubmitCheck" value="1" /> 
</form> 
<div id="formBottom">
</div> 
</div> 
</div> 
</body>
</html>`

Please can someone advise what needs changing?