Think of the term "f(x)" as a single variable, then solve for the term "x", manipulate the equation like how one would manipulate a literal equation, by inverse operations.
f(x) = 15x - 1
+1 +1
f(x) + 1 = 15x
/15 /15
(f(x) + 1)/15 = x
This means that (denote in the way one would denote an inverse of a function);
