magnitude and direction?

Question

I am having problems with this problem can someone tell me what I am doing wrong I can not find what U =
the question
a 1500 kg car is traveling with a speed of 15 m/s.
what is the magnitude of the horizontal net force that is required to bring the car to a halt in a distance of 50.0 m?
this is what I have but it does not add up!
F=ma
a can be obtained from V squared - U squared =
-2as
V = 0 final velocity
a = U squared / 2s
F = m (U squared / 2s)
m = 1500 kg
U = ?
s = distance = 50.0 m

Answer 1

t = 50/15 = 3.333 sec
a = 15/3.333 = 4.5 m/s^2
f = 1500*4.5 = 6750 N

Answer 2

v^2 = u^2 + 2as
v=0 m/s, u=15 m/s, s=50 m
Substituting,
0 = 225 + 100a, or a = -2.25 m/s^2 (decceleration)
F = ma
m=1500 kg, a= -2.25 m/s^2
Hence, F= -1500 x 2.25 = -3375 N
The magnitude of the force required is 3375 N. The negative sign indicates that its direction is opposite to the direction in which the car is moving.

Answer 3

U is given as 15 m/s
a = 225/(2*50) = 2.25m/s^2
F = ma = 1,500*2.25 = 3,375 N