The given expression is
[tex]f(x)=9x+54[/tex]The expression f(x) = 18 means that we have to evaluate the function f(x) when y = 18. Let's replace this value and solve for x.
[tex]\begin{gathered} 8=9x+54 \\ 18-54=9x \\ x=\frac{-36}{9}=-4 \end{gathered}[/tex]