Answer :
let x be the hours at grocery store and y at carwash. We get that
[tex]\begin{gathered} \begin{cases}x+y=30 \\ 12x+14y=396\end{cases}\rightarrow\begin{cases}x=30-y \\ 12x+14y=396\end{cases}\rightarrow \\ 12(30-y)+14y=396\rightarrow360-12y+14y=396\rightarrow \\ 2y=396-360=36 \\ y=18\rightarrow x=12 \end{gathered}[/tex]