(a) -3
6x2 – xy + 4cy2 = 0, 3x + 4y = 0
The other line may be (2x + by)
(3x + 4y) (2x + by) = 6x2 – xy + 4cy2
6x2 + 3xby + 8xy + 4by2 = 6x2 – xy + 4cy2
6x2 + xy (3b + 8) + 4by2 = 6x2 – xy + 4cy2 paring, 3b + 8 = -1
3b = -9 ⇒ b = -3
4b = 4c ⇒ 4(-3) = 4c
-12 = 4c ⇒ c = -3