Assuming constant acceleration a, the object has undergoes an acceleration of
a = (50 m/s - 100 m/s) / (25 s) = -2 m/s²
Then the net force has a magnitude F such that, by Newton's second law,
F = (75.0 kg) a
F = (75.0 kg) (-2 m/s²)
F = -150 N
meaning the object is acted upon by a net force of 150 N in the direction opposite the initial direction in which the object is moving.
