As the The average rate of flow is given by
(1/(R - 0)) * ∫(r = 0 to R) k(R^2 − r^2) dr
So what we do is that we proceed like this:
= (k/R) * ∫(r = 0 to R) (R^2 − r^2) dr
= (k/R) * (R^2 r − r^3/3) {for r = 0 to R}
= (k/R) * (R^2 * R − R^3/3) - 0
= (k/R) * (2R^3/3)
= (2k/3) R^2.
I hope this can help you for good