Nineteenth-century physician Jean Louis Marie Poiseuille discovered that the rate of change of the velocity of blood $r$ cm from the central axis of an artery (in centimeters per second per centimeter) is given by
$$a(r)=-k r$$ where $k$ is a constant. If the radius of an artery is $R \mathrm{cm}$, find an expression for the velocity of blood as a function of $r$ (see the accompanying figure).