Take "to the right" to the be the positive horizontal direction. Then the net force on the box in the horizontal is
∑ F = 225 N - 100 N = 125 N
and by Newton's second law,
∑ F = ma ↔ 125 N = (250 kg) a
where a is the acceleration of the box. Solve for a :
a = (125 N) / (250 kg) = 0.5 m/s²
