I suggest you first use a kinematic relationship to get the acceleration, then use Newton's 2nd law to get the force.
Kinematic relationship to get acceleration:
vf2 - vi2 = 2ax
vf = final speed = 0 m/s (comes to a full stop)
vi = initial speed = 15.0 m/s
x = distance over which the accelerating force acts to bring about a full stop = 50.0 m
a = acceleration
a = -vi2/(2x) = -(15.0 m/s)2 / [2(50.0 m)] = -2.25 m/s2
The negative sign indicates it is a deceleration.
Newton's 2nd law:
F = ma = (1580 kg)(-2.25 m/s2) = -3555 N
The negative sign indicates the force is opposing the direction of motion. The magnitude of the force is
|F| = 3555 N