[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\\
variation
\end{array}\\\\
-------------------------------\\\\
\begin{cases}
y=8\\
x=2
\end{cases}\implies 8=k2\implies \cfrac{8}{2}=k\implies 4=k
\\\\\\
thus\qquad \qquad \boxed{y=4x}[/tex]
