we are given
number of branches is
[tex]B=y-3[/tex]
number of leaves on each branch
[tex]n=2B^2-B[/tex]
the average number of leaves = (number of branches)*(number of leaves on each branch)
so, we can plug values
we get
[tex]f(y)=B*n[/tex]
now, we can plug values
[tex]f(y)=(y-3)*(2(y-3)^2-(y-3))[/tex]
now, we can simplify it
[tex]f(y)=2y^3-19y^2+60y-63[/tex]............Answer