# Let f(x)=9x-8, how do you find (fof)(5)?

Assuming you mean $f \left(5\right)$, then $f \left(5\right) = 37$
If we have $f \left(x\right)$ as some transformation applied to $x$, then $f \left(a\right)$ will be the same transformation but applied to $a$.
So if $f \left(x\right) = 2 {x}^{2} + 9$, then $f \left(a\right) = 2 {a}^{2} + 9$. And if we say $a = 5$, then $f \left(a\right) = 2 {\left(5\right)}^{2} + 9 = 59$
So, using this principle, $f \left(5\right) = 9 \left(5\right) - 8 = 37$