The <i><b>inverse</b></i> function of a linear function of slope m and y-intercept b is 
&fnof;<sup><i>-1</i></sup>(x) = (1/m)x - b/m.
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Inverse functions are reflections of each other across 
(are symmetric about) the main diagonal y=x.
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Their point of intersection is on the main diagonal y=x. 
The x and y coordinates are b/(1-m).
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<img src="linearFinv.png" width="200" height="200">
<img src="linearFinv2.png" width="200" height="200">
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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.