The equation of circles are given as, x2+y2=b2 (x−a)2+y2=b2 The equation of the comment tangent for 1 st is, y=mx−b√1+m2 The equation of the common tangent for 2 nd circle is, y=m(x−a)+b√1+m2 Equate the common tangents, mx−b√1+m2=mx−am+b√1+m2 2b√1+m2=am a2m2=4b2+4b2m2 a2m2=4b2+4b2m2 m2(a2−4b2)=4b2 The value of m is, m=