To be perpendicular, the slope of the new line has to have the reciprocal of the original slope.
Do this by flipping the slope (ifi t's a whole number make it a fraction) and change the sign.
3 ⇒ -1/3
Input x and y and solve for b.
5 = -1/3(-6) + b
Simplify.
5 = 2 + b
Subtract 2 from both sides.
3 = b
y = -1/3x + 3
