I'm going to think of the car as traveling from east (-) to west (+) and the truck from south (-) to north (+). First think how these movements are related. Since the roads are at right angles, we can use a right triangle and the pythagorean theorem. Let's call the car's road a, and the truck's road b. The distance between them we'll call c. We need to know how fast they are separating, so we need to know how c is changing over time or

To get we need to take the derivative with respect to t.

Now plug in the information that you know. Note that the car has moved from 120 ft away to 60 ft away in the 2 seconds, etc.

Solve for