TITLE: The equation of a line reflected about another line
QUESTION [8 upvotes]: I need to find the equation of this reflected line:

REPLY [4 votes]: The General Formula for finding the equation of the line when the line $Ax+By=C$ is reflected at the line $Dx+Ey=F$. 
$$y-h=(\frac{AE^2-AD^2-2BDE}{BE^2-BD^2+2ADE})(x-k)$$
where
$$k=\frac{CE-BF}{AE-BD}$$
$$h=\frac{CD-AF}{BD-AE}$$
$h$ and $k$ is simply the intersection of the lines although if the two lines are parallel then it $h=0, k=0$,
This is easily derived by the use of perpendicular slopes and midpoint formula.