where N is the normal reaction force of the road on the car and the downwards direction is positive.
But Fnet = mv^2/r = 7500.
Therefore 7500 = 14700 - N => N = 7200 Newton.
From Newton III, size of force exerted by car on road = 7200 Newton.
b) When the car is on the point of "lifting up off the bridge" N = 0.
Therefore mv^2/r = mg
=> 1500 v^2/20 = 14700 => v = ....