ok, so you should know that

Here, the total time for the faster car 3 hrs. the total time for the slower car is 3.5 hrs.

Now, let the average speed of the faster car be , then the average speed of the slower car is .

also, they travel the same distance, , which is yet an unknown.

so, we have:

...........................(1) this is for the faster car

and

..............(2) this is for the slower car

you have two equations and two unknowns, all you want to find is and you will know the average speed of the faster car, and hence, by subtracting 10, the average speed of the slower car