$($a$)$ The volume flow rate in an artery supplying the brain is $3\mathrm{.}6$ x $10^-6$ m$^3/$s$.$ If the radius of the artery is $5\mathrm{.}1$ mm$,$ determine the average blood speed. $($b$)$ Find the average blood speed as a constriction in the artery if the constriction reduces the radius by a factor of $3.$ Assume that the volume flow rate is the same as that in part $($a$).$