Calc, optimization problems help, ive been on this forever?

The human cough is intended to increase the flow of air to the lungs, by dislodging any particles blocking the windpipe and changing the radius of the pipe. Suppose a windpipe under no pressure has radius r0. The velocity of air through the windpipe at radius r is approximately V(r) = cr2(r0 - r) for some constant c. Find the radius that maximizes the velocity of air through the windpipe. Does this mean the windpipe expands or contracts?