The average rate of blood flow is given by
[tex]\displaystyle\frac1{R-0}\int_0^Rv(r)\,\mathrm dr=\frac kR\int_0^R(R^2-r^2)\,\mathrm dr[/tex]
[tex]=\dfrac kR\left(R^2r-\dfrac13r^3\right)\bigg|_{r=0}^{r=R}[/tex]
[tex]=\dfrac kR\left(R^3-\dfrac13R^3\right)[/tex]
[tex]=\dfrac{2kR^2}3[/tex]