Given, S=x2+y2+2x+17y+4=0 S′′=x2+y2+7x+6y+11=0 S=x2+y2−x+22y+3=0 S−S′=5x−11y+7=0 ...(i) S′−S′′=8x−16y+8=0 x−2y+1=0 ...(ii) Solving Eqs. (i) and (ii), we get (3,2) ∴ Radical centre of circle is (3,2). Length of tanget from (3,2) to S is √St =√(3)2+(2)2+2(3)+17(2)+4=√57