Dash is correct – you have to make sure all of your units line up, which is best done by showing all work.
Newton’s Second Law: F = ma
The gravitational acceleration is g = 9.8 m/s
Centripetal acceleration is a = v^2/r
So to keep that car on track, you want to balance the centripetal acceleration with gravitational acceleration, so: 9.8 = v^2/r
Your r is known: 50ft, which is 15.24m, so v^2 = 9.8 * 15.24, and v = sqrt(9.8 * 15.24) = 12.22 m/s.
Converting 12.22 m/s into mph and you get 27.3.