Please follow below specified steps to determine what exact problem is there
1)Check that DevExpress.Web.v7.2 dll
is present in system with specified version ,
if above dll is present in your system then do one thing paste this dll is your solution folder or bin folder of application where exactly you are getting this error.
2)If dll is present in system as well as in solution or bin directory then check machine configurion (your machine is having 32 bit operating system or 64 bit operating system)and please also check that your dll DevExpress.Web.v7.2 dll is 32 bit version or 64 bit version.you can check version with utility http://www.dependencywalker.com/
which will tell what is the version of the dll.
3)you can check also that any dependent dll is missing for DevExpress.Web.v7.2 dll