How do you get r and can you justify each step.

To get r you have to divide:
-8r=64

-8
You would divide by -8 because you are trying to get r by itself and make sure when you divide one side divide the other one:
-8r=64
r= -8
-8
You would get -8 when you divide 64/-8 because a negative and a negative gives you a positive hope this helps a bunch :