What is the difference in miles between the two trains after

What is the difference, in miles, between the two trains after each train travels for 6 hours?
Train L and train m leave a station at the same time heading in opposite directions. train l is going north at an average speed of 75 miles per hour. train m is giong south at an average speed of 60 mph?

Solution

they are moving apart at 135 mph
D=rt
D=135*6
D=810 miles