Answer:
[tex]\boxed{3}[/tex]
Step-by-step explanation:
The constant of proportionality is the ratio of corresponding sides
[tex]\text{Ratio} = \dfrac{15}{5} = \dfrac{36}{12} = \dfrac{39}{13} =\mathbf{3}\\\text{The constant of proportionality is $\boxed{\mathbf{3}}$}[/tex]
