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

• Foot of perpendicular F(h, k): (h - x1)/a = (k - y1)/b = -(ax1 + by1 + c)/(a^2 + b^2)
• Image Q(h, k): (h - x1)/a = (k - y1)/b = -2(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.