7 - 8r = 5 - 7r + 3 - 6 (given)
7 - 8r = 2 - 7r (combine similar terms on both sides)
-8r + 7r = 2 - 7 (transpose all terms with variable r to the left side, and constants to the right side)
-r = -5 (combine similar terms on both sides)
r = 5 (multiply both sides by -1)