Acar accelerated from 88 feet per second to 220 fps in exactly 3 seconds. assuming the accerlation was constant, what was the car's acceleration, in feet per second, from 88 fps, to 220 fps?

Acceleration is the derivative of velocity, so it is a measure of the change in velocity over time. Dimensional units reflect this as fps (feet per second) for velocity and fps^2 (fps per second). Here, acceleration is simply given as the change in velocity divided by the time. 
This works as long as all your units are equivalent (i.e. if time was in minutes and velocity was in f/s, you would need to convert first).

The coefficient, 5 represents the length in relation to the problem.