How do I integrate the following mean value of this probability function?

Find the mean value of the following probability function: p(x)=(1/64)*x*e^(-x/8). So for the mean value, I set up an integral from 0 to infinity, multiplying by x, which gives: (1/64) Int(from 0->Inf) (x^2)*e^(-x/8)dx. But, I have tried substitution by parts, and I just can't seem to get it to work.