Chapter 4 - Exponential and Logarithmic Functions - Section 4.2 One-to-One Functions; Inverse Functions - 4.2 Assess Your Understanding - Page 290: 7

$f^{-1} (8) =3$

By the definition of an inverse one-to-one function, if $f(x)=y$, then we have $f^{-1} (y)=x$. For each $x$, there must be a unique value for $f(x)$. Therefore, when $f(3)=8 $, then its inverse can be expressed as: $f^{-1} (8) =3$ Note, that the difference between the function and its inverse is that the values for $x$ and $y$ get switched.