Question

Is r = -1 a solution to the equation 6(r – 4) = 30r

Answer 1

6(-1 - 4) = 30(-1)
6(-5) = -30
-30 = -30
so -1 is a solution

Answer 2

You can try to insert r=-1: 

6(-1-4) = 30*-1
-6-24=-30
-30=-30

So, yes r = -1 is a solution to the equation.