I have three points:
$P = (2,5)$
$Q = (12,5)$
$R = (8,-7)$
I need to find the equation of the line thru $R$ which is perpendicular to $PQ$.
How do I do this? $PQ$'s gradient is $\frac{5-5}{12-2}=0$ so the line perpendicular to that has what kind of gradient?