a fullback having a mass of 9.0x10^1 kg is running at 4.5 m/s [E]. He is hit by a linebacker and 1.2 s later is moving backwards at 1.5 m/s [W].Find:

Athe impulse imparted to the fullback B) the average force exerted on the fullback by the linebacker

my work: impulse is p so p= p2-p2 p=m2v2-m1v1 but its all wrong because the answer is 5.4 x 10^2 kg.m/s [w] apparently. Please help!