Can someone explain this (4+5)+2=9+2

Which of the following is correct interpretation of the expression -4+13

Two cars which are 40 miles apart start moving simultaneously in the same direction with constant speeds. if the speed of the car which is behind is 56 mph and the speed of the other car is 48 mph, how many hours will it take for the car which is behind to catch up the car ahead of it?

Sole the equation 6n + 9 = -3n - 21 - 6 for n