persamaan garis lulus yang bergadien -3 dan melalui titik (1,2)adalah

m1 x m2 = -1
-3 x m2 = -1
m2 = -1/-3 = 1/3

Y-Y1= M2(X-X1)
Y - 2 = 1/3 (X - 1)
3Y -6 = X - 1
3Y - X -6 + 1 = 0
3Y - X -5 = 0
X - 3Y + 5 = 0