Question

G(r)=−5r+13 g(3)= I'm still confused on how to do this

Answer 1

If you're solving for g then move all terms to the left side and set equal to zero. Then set each factor equal to zero.

And your answer becomes $g=\frac{5r}{r-39}$

If you're solving for r then then move all terms to the left side and set equal to zero. Then set each factor equal to zero and your answer is $r=\frac{39g}{g-5}$