Inverse functions

First, to find $f^{-1}(x)$, we replace $x$ with $f^{-1}(x)$.

$f(f^{-1}(x)) = 5f^{-1}(x) - 12\\ x = 5f^{-1}(x) - 12\\ f^{-1}(x) = \dfrac{x + 12}5$

Also, what is f(x - 1)? We need to find it as well! Whenever we want to find $f(\triangle)$, we replace x by $(\triangle)$. In this case, replace x with (x - 1).

$f(x - 1) = 5(x - 1) - 12 = 5x - 17$

Then, the equation is just $\dfrac{x + 12}5 = 5x - 17$. I believe you can solve the linear equation on your own.