Mark M. answered 11/22/19
Retired Math prof with teaching and tutoring experience in trig.
Average value of y = f(x) from x = a to x = b is (1/(b-a))∫from a to b f(x)dx
So, we have (1/(3-1))∫(from 1 to 3) xex^2dx
= (1/2)[(1/2)ex^2]from1 to 3 = (1/4)(e9 - e)