Aboat of mass 225 kg drifts along a river at a speed of 21 m/s to the west. what impulse is required to decrease the speed of the boat to 15 m/s to the west?

The impulse required to decrease the speed of the boat is equal to the variation of momentum of the boat:

where
m=225 kg is the mass of the boat
is the variation of velocity of the boat
By substituting the numbers into the first equation, we find the impulse:

and the negative sign means the direction of the impulse is against the direction of motion of the boat.

Ithink the answer would be a