$$49 x ^ { 2 } - b = ( 7 x + \frac { 1 } { 2 } ) ( 7 x - \frac { 1 } { 2 } ) , b$$
$b=\frac{1}{4}=0.25$
$x\in \mathrm{C}$
$b = \frac{1}{4} = 0.25$
$x\in \mathrm{R}$
$b = \frac{1}{4} = 0.25$