56 250 N

Initially:
PE = 0 J (the cars height is zero)
KE = 0.56000(20)^2 = 1 200 000 J
Finally:
PE = 0 J (the cars height is zero)
KE = 0.56000(5)^2 = 75 000 J
The work done is F 20 cos 180 = -20F
Using the equation,
TMEi + Wext = TMEf
1 200 000 J + (-20F) = 75 000 J
Using some algebra, it can be shown that 20F = 1 125 000 and so F = 56 250 N