[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}\\\\
-------------------------------\\\\[/tex]
[tex]\bf \textit{\underline{y} varies directly with \underline{x}}\implies y=kx
\\\\\\
\textit{we also know that }
\begin{cases}
y=-10\\
x=2
\end{cases}\implies -10=k2\implies \cfrac{-10}{2}=k
\\\\\\
-5=k\qquad thus\qquad \boxed{y=-5x}\\\\
-------------------------------\\\\
\textit{what's \underline{y} when \underline{x} is 12?}\qquad y=-5(12)[/tex]
