to get the inverse, first off do a quick switch on the variables, and then solve for "y".
[tex]\bf f(x)=y=-3x+3\qquad \qquad inverse\implies \boxed{x}=-3\boxed{y}+3
\\\\\\
x-3=-3y\implies \cfrac{x-3}{-3}=y\implies \cfrac{3-x}{3}=y\impliedby f^{-1}(x)[/tex]
