Baseball pitching machines such as shown in (figure 1) are used to fire baseballs toward a batter for hitting practice. in one kind of machine, the ball is fed between two wheels that are rapidly rotating in opposite directions; as the ball is pulled in between the wheels, it rapidly accelerates up to the speed of the wheels' rims, at which point it is ejected.

for a machine with 35-cm-diameter wheels, what rotational frequency (in rpm) do the wheels need to pitch a 90 mph fastball?
express your answer in revolutions per minute.