There are three forces acting on the car - gravity, a normal force N by the road, and a frictional force by the road. Since the car is accelerating at (the centripetal acceleration), these must add up to a force acting horizontally. Since we are looking for the minimum coefficient of friction, set the frictional force to be the maximum possible, given by where is the coefficent of friction.
So we have:
(straight down) plus
(perpendicular to the road, up and toward the center) plus
(parallel to the road, up and away from the center) equals
(horizontal toward the center)
First, calculate by looking at the components perpendicular to the road:
Then calculate (and then ) by looking at the components parallel to the road:
You should be able to take it from there. If you're still having trouble, go ahead and post again in this thread.