Hard Physics Problem - Angular Momentum?

[Nik]

Water drives a waterwheel (or turbine) of radius 3.5 m. The water enters at a speed v1 = 7.5 m/s and exits from the waterwheel at a speed v2 = 4.0 m/s.
a) If 84 kg of water passes through per second, what is the rate at which the water delivers angular momentum to the waterwheel? [find: delta L/delta t = net torque]
b) If the water causes the waterwheel to make one revolution every 5.3 s, how much power is delivered to the wheel?