Remember that the net momentum before a collision is equal to the net momentum after a collision. Let momentum= p, mass= m, and v= velocity. Even though a pound is not a measurement unit of mass, it is linearly correlated with it, so let's use pounds to measure measure mass. Note that if the two cars couple after the collision that their velocity will be the same after the collision. p=mv. Net p= (m1v1)+(m2v2) ... (before the collision)= (m1v1)+(m2v2) ... (after the collision. Now plug the numbers in: (12,600 lbs. x 7.4 m/s) + (6,490 lbs. x 0 m/s)= v(12,600 + 6,490). Solve for v and you will get their velocity after the collision. Sorry, I don't have time now to answer the impulse part. Good Luck. P.S. impulse is not measured in lbs. or lbs./sec.
