To solve this, you need to isolate/get the variable "r" by itself in the equation:
-1 = -3r + 2r Combine like terms(terms that have the same variable and power/exponent.) The only like terms you can combine are -3r and 2r
-1 = -r Divide -1 on both sides to get "r" by itself
(two negative signs cancel each other out and become positive)
1 = r or r = 1