Your question says what is the constant of proportionality of the equation below
[tex]y=4.5x^3[/tex]A proportional relationship can be represented as follows
[tex]y=kx[/tex]Where K is the constant of proportionality, base on your question, the constant of proportionality is 4.5 .
[tex]k=4.5=\frac{9}{2}[/tex]