[tex]\bf 3x^2=4y=12\quad
\begin{cases}
3x^2=4y\\
4y=12\\
-----\\
3x^2=12
\end{cases}
\begin{cases}
3x^2=12\\
x^2=\frac{12}{3}\\
x^2=4\\
x=\sqrt{4}\\
\boxed{x=2}
\end{cases}\quad
\begin{cases}
4y=12\\
y=\frac{12}{4}\\
\boxed{y=3}
\end{cases}
\\\\\\
x^2y\implies (2)^2(3)\implies 4\cdot 3\implies 12[/tex]