Also, to make it 6m wide at a height, we have to set f(3)=8 and f(-3)=8. Now we have:

[tex]c=15 \\ 9a+3b+c=8 \\ 9a-3b+c=8[/tex]

We could easily notice that b=0 and c=15, let's find a: 9a+15=8 or 9a=-7 or [tex]a=-\frac{7}{9}[/tex]

The function is [tex]f(x)=-\frac{7}9x^2+15[/tex]