Acceleration at 50m/sec² : means every second the velocity increases by 50 m/sec. In 3 seconds the velocity increases by 150 m/s. So total will be 160 m/sec.
the formula is
V = final velocity = U initial velocity + acceleration a * time duration t
V = 10 m/s + 50 m/sec² * 3 sec = 160 m/sec