2x(g1-g2)+2y(f1-f2)+c1-c2 = 0
This is for circles
Circle 1 (termed as S1) x² + y²+2g1x+2f1y+c = 0
Circle 2 (termed as S2) x² + y²+2g2x+2f2y+c = 0
Length of the common chord = 2√ (C1P²-C1M²)
C1P = radius of circle 1
C1M = length of the perpendicular from the centre C1 to the common chord PQ.