3x - 9y = 15
-9y = -3x + 15
y = (-3/-9)x + (-15/9)
y = 1/3x - 5/3....slope here is 1/3. A perpendicular line will have a negative reciprocal slope.All that means is " flip " the slope and change the sign.
So our perpendicular slope will be : -3 ( I flipped 1/3 and it became 3/1 or just 3...I then changed the sign and it became -3)
so the perpendicular slope is -3