Integration Problem for Calculus Homework

I'm having trouble with the following problem from my homework for a Calc. II course:

Quote:

The velocity *v *of the flow of blood at a distance *r *from the central axis of an artery of radius *R* is,

$\displaystyle v = k(R^2 - r^2)$

where *k *is the constant of proportionality. Find the average rate of flow of blood along a radius of the artery. (Use 0 and *R* as the limits of integration.)

Any help would be really appreciated, thanks.

EDIT: I should probably clarify: I know how to use integration in order to find the average value of a function over a range. It's integrating the function that I'm stuck on.