(b)
Given, $|\overrightarrow{\mathbf{a}} \times \overrightarrow{\mathbf{b}}|^{2} + |\overrightarrow{\mathbf{a}} \bullet \overrightarrow{\mathbf{b}}|^{2} = 144$
$\Longrightarrow \left| \overrightarrow{\mathbf{a}} \right|^{2}{+ \left| \overrightarrow{\mathbf{b}} \right|}^{2}\left( \sin^{2}\theta + \cos^{2}\theta \right) = 144$
$\Longrightarrow 16\left| \overrightarrow{\mathbf{b}} \right|^{2} = 144$
$\Longrightarrow \left| \overrightarrow{\mathbf{b}} \right| = 3$