Help with integration problem

I have the solution for this integral, but I'm not getting the same answer.

This is the integral:

https://i.imgur.com/bxmhDxT.png

and the solution is:

https://i.imgur.com/y2iEysV.png

Whenever I try it, I come up with it being equal to zero if n != m, which is wrong.

Edit: In the solution for n = m, the m in the denominator is actually mass, which I used the letter mu for Edit: Also, H is an operator, not a multiplicative factor