Let x be the average speed of the slower car, in kilometers per hour.


Then the faster car's average speed is (x+3) km/h.


The "time" equation is


     = .      { " in the same amount of time " )


Solve it for x.


For it, cross multiply to get


    310*x = 215*(x+3)

    310x = 215x + 645

    310x - 215x = 645

    95x = 645

    x =  = 6.8 kilometers per hour (approximately).


Very strange answer, for the car' speed - but such are the input data in your post.