Let ax + by + c = 0 is a straight line. If a perpendicular line is drawn from any point on the line to this straight line,

then the point of intersection of the given striaght line and its perpendicular is called the foot of the corresponding perpendicular. 

Let B(x₂ , y₂ ) is the foot of the perpendicular drawn from the point A(x₁ , y₁ ) on the line  ax+ by + c = 0

Then   (x₂ - x₁ )/a = (y₂ - y₁ )/b = -(ax₁ + by₁ + c)/(a² + b² )