[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 also know that }
\begin{cases}
y=6\\
x=-2
\end{cases}\implies 6=k(-2)\implies \cfrac{6}{-2}=k
\\\\\\
-3=k\qquad thus\qquad \boxed{y=-3x}
\\\\\\
\textit{now, when y = 15, what is \underline{x}?}\qquad 15=-3x\implies \cfrac{15}{-3}=x[/tex]
