Answer :
[tex]S=-\frac{1}{2}gt^2+vt\ \ \ |Add\ \frac{1}{2}gt^2\ to\ both\ sides\\\\
S+\frac{1}{2}gt^2=vt\\\\
\frac{2s}{2}+\frac{gt^2}{2}=vt\ \ \ |Divide\ by\ t\ both\ sides\\\\
\frac{2s+gt^2}{2t}=v\\\\Asnwer\ is\ v=\frac{2s+gt^2}{2t}
[/tex]