For finding the inverse of a function you have to write it as an ecuation.

Invert the values of x and y.

Our objetive now in this new equation is to clear y. First, subtract 7 in both sides of the equation.

Then, divide both sides by 4.

Finally, invert the values of x and y again and put the equation like a function.

This is correct because f(x) = x is the axis of symmetry for both functions (green one).