Answer
$$s=-\frac{gr}{r-g};\quad \:g\ne \:r$$
Work Step by Step
We solve the equation by using the rules of Algebra to isolate the variable:
$$ r\left(g+s\right)=gs\\ gr+sr=gs\\ sr=gs-gr\\ s\left(r-g\right)=-gr \\ s=-\frac{gr}{r-g};\quad \:g\ne \:r$$