I am trying to do this: .
I set: . . . . . Good!
I am stuck at this point now: .
We have: .
Then let: .
A minor correction:
You skipped a factor of 2. Then follow Soroban's suggestions.
I've seen at least three of these examples from you now and I'm getting the impression you are having some troubles with the actual substitution. My advice is for you to slow down and double check your work after you put the substitution in the integral, because you have a tendency to miss multiplicative factors.