Direct proportionality:
[tex]y=kx[/tex]
k - the constant of proportionality
[tex]y=2 \hbox{ when } x=3 \\ \Downarrow \\
y=kx \\ 2=3k \\
k=\frac{2}{3} \\ \\
y=? \hbox{ when } x=9 \\
\Downarrow \\
y=kx \\ y=\frac{2}{3} \times 9 \\
y=2 \times 3\\
y=6 \\ \\
\boxed{y=6 \hbox{ when } x=9}[/tex]
