Equation of normal of the parabola y2=4a(x+c) is ‌y=m(x+c)−2am−am3 ‌⇒y=mx+mc−2am−am3 Similarly, equation of normal for the parabola y2=4bx is: y=mx−2bm−bm3 As the normal is common, hence ‌mc−2am−am3=−2bm−bm3 ‌(a−b)m3=m(c+2b−2a) ‌⇒m2=‌