'Impulse' is a change in momentum.
Momentum = (mass) x (velocity)
Final momentum = (45 x12) = 540 kilogram-meters/second
Original momentum = (45 x 15) = 675 kg-m/sec
Change in momentum = -135 kg-m/sec
The impulse required is 135 kg-m/sec in the direction
opposite to the way the object is moving.