[tex]\bf \begin{array}{cccccclllll}
\textit{something}&&\textit{varies directly to}&&\textit{something else}\\ \quad \\
\textit{something}&=&{{ \textit{some value}}}&\cdot &\textit{something else}\\ \quad \\
y&=&{{ K}}&\cdot&x
\\ \\
&& y={{ K }}x
\end{array}
\\ \quad \\
\begin{cases}
y=4\\
x=12
\end{cases}\implies y=Kx\implies 4=K\cdot 12[/tex]
solve for "K", to see what "K" is, or the "constant of variation",
once you found K, plug it back in y=Kx, and that's the equation
