I need to prove:

Where

, is a function of

and

, and is merely piecewise continuous, so it doesn't have to be differentiable at certain points, and

is an arbitrary function also of

and

, which is infinitely differentiable and has compact support.

My Attempt:

I tried to change variables by using

and

, and I later realized that it basically became an exercise in trying to prove

which didn't work out.

Can anyone give me a hint of some kind on how to try and proceed? Any suggestions would be greatly appreciated. Thanks in advance.