[tex]\bf \qquad \qquad \textit{direct proportional variation}\\\\
\textit{\underline{y} varies directly with \underline{x}}\qquad \qquad y=kx\impliedby
\begin{array}{llll}
k=constant\ of\\
\qquad variation
\end{array}\\\\
-------------------------------\\\\
\textit{we know that }
\begin{cases}
x=3\\
y=12
\end{cases}\implies 12=k3\implies \cfrac{12}{3}=k\implies 4=k
\\\\\\
thus\qquad \boxed{y=4x}
\\\\\\
\textit{when x = 12, what is \underline{y}?}\qquad \qquad y=4(12)[/tex]
