4a2+b2+2c2 + 4ab – 6ac – 3bc = 0 = (2a+b)2 - 3 (2a + b) + 2c2 = 0 ⇒ (2a + b - 2c) (2a + b - c) = 0 ⇒ c= 2a + b or c = a + 21 b The equation of the family of lines is a (x + 2) + b (y + 1) = 0 or a (x + 1) + b (y+21) = 0 giving the point of concurrence (–2, –1) or (−1,21)