To obtain the inverse of the function, simply switch the X and y variables for each other, and solve for the new y variable, to find the new function.
F(X) = -3x + 3
Y = -3X + 3
X = -3y + 3
-3y = X - 3
Y = -1/3X + 1.
F^-1(X) = -1/3(-1) + 1
F^-1(X) = 1/3 +3/3
F^-1(X) = 4/3.
I believe this would be the correct solution.