For point P(x1, y1) and line ax + by + c = 0:

• Foot of perpendicular F(h, k): $\frac{h - x1}{a}$ = $\frac{k - y1}{b}$ = -$\frac{ax1 + by1 + c}{(a^2 + b^2)}$
• Image Q(h, k): $\frac{h - x1}{a}$ = $\frac{k - y1}{b}$ = -2$\frac{ax1 + by1 + c}{(a^2 + b^2)}$

The image formula has a factor of -2 (foot has -1). The foot F is the midpoint of P and its image Q.