Answer
$a=\dfrac{2s-2vt}{t^2}$
Work Step by Step
Subtract $vt$ from both sides to find:
$s-vt=\frac{1}{2}at^2$
Multiply $2$ to both sides to find:
$2(s-vt)= 2 \cdot \frac{1}{2}at^2
\\2s-2vt=at^2$
Divide both sides by $t^2$ to find:
$\dfrac{2s-2vt}{t^2}=\dfrac{at^2}{t^2}
\\\dfrac{2s-2vt}{t^2}=a
\\a=\dfrac{2s-2vt}{t^2}$
