First, I suspect there is a typo in your statement of the problem. Your integrand is [tex]x^2[tex] but the integral with respect to x!
I suspect you meant . Is that correct? Assuming it is-
I'm not sure why you would differentiate. If you just go ahead and integrate so that .
So your equation is
Solve that for f(x). (f(x) is just a number. This is exactly the same as solving for x.)