The Banked Turn with Friction

A speedway 13 m wide is banked on a curve so that the outside edge is 5 m higher than the inside edge. A racecar goes around the curve at 50 m/sec (a little better than 110 mph). The coefficient of static friction of the racecar's tires is $\mu_s = 0.6$ (good tires). a) Find an algebraic expression for the smallest radius the curve could have such that the car can make the turn; b) IF YOU HAVE TIME you can evaluate this expression for extra credit.