Coefficient of x7 in (x2+bx1)11
=Cr11(x2)11−r⋅(bx1)r
=Cr11x22−3r⋅br1
So, 22−3r=7
⇒r=5
So, the term is C511⋅b51⋅x7
Coefficient of x−7 in (x−bx21)11
=Cr11(x)11−r⋅(−bx21)r
=Cr11x11−3r⋅br(−1)r
So, 11−3r=−7∴r=6
So, the term is C611⋅b61x−7
Hence, C511⋅b51=C611⋅b61
Since b=0∴b=1