smc

Solve for $t$ in the first equation: $$V-V_0=gt$$ $$t=\frac{V-V_0}{g}$$ Then use the second equation to solve for $g:$ $$2(S-V_{0}t)=g{t}^2$$ $$g=\frac{2(S-V_{0}t)}{t^2}$$ Plug in $g$ in $t$ to get $\boxed{\frac{2S}{V+V_0}}.$