so f(x) is equal to them saying y = 3(3x+4)
here is an example of how to solve an inverse function
f(x)=3(3x+4) x=93
switch x and y which will give you this
x = 3(3y+4)
substitute 93 in for x
93=3(3y+4)
then solve
93=9y+12
subtract 12 from both sides which will give you
81=9y
then divide by 9 on both sides to get
9=y