You probably rewrote the expression as $\displaystyle{\sqrt[{{3}}]{{{{\left(-{1}\right)}}^{{3}}\cdot{{2}}^{{3}}\cdot{12}\cdot{{b}}^{{3}}\cdot{{b}}^{{2}}}}}\usepackage{cmbright}\Huge\$ , and correctly took the cube roots of the perfect cubes, but you left out the remaining factors. The correct answer is: $\displaystyle-{2}{b}{\sqrt[{{3}}]{{{12}{{b}}^{{2}}}}}\usepackage{cmbright}\Huge\$