The inverse function of a linear function of slope m and y-intercept b is ƒ-1(x) = (1/m)x - b/m.
Inverse functions are reflections of each other across (are symmetric about) the main diagonal y=x.
Their point of intersection is on the main diagonal y=x. The x and y coordinates are b/(1-m).

A function has only one inverse (if it has one at all [the function has to be monotonically increasing or decreasing, i.e. it has to pass the horizontal line test, no horizontal line can touch the function's graph more than once]; but every linear function has an inverse). y=x and y=-x are their own inverses.