[tex]\bf \begin{array}{cccccclllll}
\textit{something}&&\textit{varies directly to}&&\textit{something else}\\ \quad \\
\textit{something}&=&{{ \textit{some value}}}&\cdot &\textit{something else}\\ \quad \\
y&=&{{ k}}&\cdot&x
\\
&& y={{k }}x
\end{array}\\\\
-------------------------------\\\\[/tex]
[tex]\bf \textit{we know that }\qquad
\begin{cases}
y=42\\
x=6
\end{cases}\implies 42=k6\implies \cfrac{42}{6}=k
\\\\\\
7=k\impliedby \textit{constant of variation}
\\\\\\
thus\qquad \qquad y=7x
\\\\\\
\textit{what is "y" when x=5?}\qquad y=7(5)\impliedby \textit{solve for "y"}
\\\\\\
\textit{what is "x" when y=28?}\qquad 28=7x\impliedby \textit{solve for "x"}[/tex]
