[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}
\stackrel{f(x)}{y}=-9\\
x=3
\end{cases}\quad -9=k3\implies \cfrac{-9}{3}=k\implies \boxed{-3=k}
\\\\\\
thus\qquad \qquad \stackrel{f(x)}{y}=-3x[/tex]
