A rod of length eight units moves such that its ends A and B always lie on the lines x – y + 2 = 0 and y + 2 = 0, respectively. If the locus of the point P, that divides the rod AB internally in the ratio 2 : 1 is 9(x2 + ay2 + βxy + γx + 28 y) – 76 = 0, then α – β – γ is equal to :