[tex]\bf \qquad \qquad \textit{direct proportional variation}\\\\
\textit{\underline{y} varies directly with \underline{x}}\qquad \qquad \stackrel{f(x)}{y}=kx\impliedby
\begin{array}{llll}
k=constant\ of\\
\qquad variation
\end{array}\\\\
-------------------------------\\\\
\textit{we also know that }
\begin{cases}
y=-18\\
x=3
\end{cases}\implies -18=k3
\\\\\\
\cfrac{-18}{3}=k\implies -6=k\qquad therefore\qquad \boxed{\stackrel{f(x)}{y}=-6x}[/tex]
