Acar has a momentum of 20,000 kg • m/s. what would the car’s momentum be if its velocity doubles?

To answer this question, it helps enormously if you know
the formula for momentum:

           Momentum = (mass) x (speed) .

Looking at the formula, you can see that momentum is directly
proportional to speed.  So if speed doubles, so does momentum.

If the car's momentum is 20,000 kg-m/s now, then after its speed
doubles, its momentum has also doubled, to 40,000 kg-m/s.