Answer
$f^{-1}(x) = \frac{x-5}{3}$
Work Step by Step
$f(x) = 3x+ 5$
Let $f(x) = y$:
$y = 3x + 5$
Switch the variables $x$ and $y$ and then solve for $y$:
$x = 3y + 5$
$x - 5 = 3y$
$y = \frac{x-5}{3}$
$f^{-1}(x) = \frac{x-5}{3}.$