In order for a relation to be a proportion, you will need that every quotient to be constant. In this case it is necessary that
[tex]\frac{12}{3}=4,\text{ }\frac{20}{5}=4,\text{ etc}[/tex]are the same. Since this is true, then it is a proportion, and the constant of proportionality is 4, that is
[tex]y=4x\text{.}[/tex]You can check this by subtitution, and his is the equation that gives you the proportion relation.
