The value of b for which the equation 2log1/25⁡(bx+28)=−log5⁡12−4x−x2 has coincident roots is

2log5⁡(bx+28)log5⁡(1/5)2=−log5⁡12−4x−x2⇒ bx+28=12−4x−x2 or  x2+(b+4)x+16=0For coincident roots,D=0⇒(b+4)2=4(16)⇒b+4=±8