The correct answer is f^-1(x)=1/2x+2.
1. Substitute y for f(x). ( y=2x+4 )
2. Interchange the variables x and y. ( x=2y-4 )
3. Swap the sides. ( 2y-4=x )
4. Move the constant to the right. ( 2y=x+4 )
5. Divide both sides by 2. ( y=1/2x+2 )
6. Use substitution to substitute f^-1(x) for y.
Thus the inverse is f^-1(x)=1/2x+2