Ok so to solve for r you need to isolate the variable. This is what you do.
6r - 12 = 6
Add 12 to both sides.
6r = 18
Divide.
r = 3
Lets check our answer.
6(3) - 12 = 6
18 - 12 = 6
6 = 6 It makes sense so r = 3 is the answer.
I hope this helps love! :)