Check the picture below.
notice the reflected gray line on the red one over the y=x line, the slope of either line across the y=x line is simply the other's upside-down.
[tex]\stackrel{\textit{slope of line "a"}}{a\implies \cfrac{a}{1}}\hspace{5em}\stackrel{\textit{slope of its reflection}}{\cfrac{1}{a}}[/tex]
