Look up pulleys problem through Khan academy and a video should pop up with a problem similar and you should be able to walk through it .
Explanation:
Momentum before = momentum after
m₁ u₁ + m₂ u₂ = m₁ v₁ + m₂ v₂
(65 kg) (0 m/s) + m (0 m/s) = (65 kg) (-3.5 m/s) + m (4 m/s)
m ≈ 57 kg
