sorry...been away from comp...
if you still need the help, i think you need to find the centripetal force (m*v^2/r) that is equal in magnitude to the weight of the car (m*g)....then, with the velocity you find there (v = sqrt(r*g) ??), you can get the kinetic energy of the car at the top of the loop...add that kinetic energy to the potential energy of the car at the top of the loop (1.67e4 or whatever it was) to get the total mechanical energy (which = total potential energy (50kg*h*g) at the top of the ramp)...from there, you can get the minimum h...i think that should work...just guessing though