Answer
$g = \frac{2s}{t^{2}}$
Work Step by Step
Our goal is to isolate g. To accomplish this we will first multiply both sides by two, then divide both sides by $t^{2}$:
$s = .5gt^{2}$
$2s = gt^{2}$
$\frac{2s}{t^{2}} = g$
$g = \frac{2s}{t^{2}}$