a direct proportional variation, is really just a slope-intercept form of equation, except that the y-intercept is always 0, and the constant of variation "k" is the slope pretty much.
[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}\\\\
-------------------------------\\\\
y=\stackrel{\stackrel{slope}{k}}{2}x\stackrel{y-intercept}{+0}\impliedby \stackrel{k}{2}\qquad \qquad \stackrel{slope}{\cfrac{2}{1}}[/tex]
