(7b-3)*(7b+3)-7b*(7b+3)=(7b)²-3²-49b²-21b=49b²-9-49b²-21b=-9-21b

при b=-2,6    -9-21*(-2,6)=-9+54,6=45,6

Verified answer

49в*в-9-49в*в-21в=-9-21в=-9+54,6=45,6