The two polygons are similar, which means there is a scale factor common to all five sides. That is, there is a factor by which you multiply the side in the red polygon which results in the corresponding side on the yellow polygon.
Observe that the side labelled 2 in the red polygon corresponds to that labelled 6 in the yellow polygon.
This means the red side is multiplied by 3 to derive the yellow side.
The side labelled y corresponds to the side labelled 15. This means;
[tex]\begin{gathered} y\times3=15 \\ y=\frac{15}{3} \\ y=5 \end{gathered}[/tex]
