p_initial = m * v_initial = 25kg * 11m/s = 275kg·m/s. After a, the momentum was p_final = m *v_final = 25kg * 8m/s = 200kg·m/s. The impulse J is: p_final - p_initial = 200kg·m/s - 275kg·m/s = -75kg·m/s.