for straight line the equation is y=mx+q or (y-yp)=m(x-xp).
For perpendicular line the equation is (y-yp)=-(1/m)(x-xp).
The angular coefficient is m in the first case and -1/m in the second case. This is the reason (video lesson 28) for the calculus to find the line who pass for P(3,1) and perpendicular to 2x+3y=5. Into this example we substitute (3,1) in 3x-2y and not in 2x+3y.
This is correct?