Note that $\displaystyle{{\left({{a}}^{{-{{3}}}}\right)}}^{{2}}={{a}}^{{-{3}{\left({2}\right)}}}\usepackage{cmbright}\Huge\$ and $\displaystyle{{\left({{b}}^{{4}}\right)}}^{{2}}={{b}}^{{{4}{\left({2}\right)}}}\usepackage{cmbright}\Huge\$ . Does this help you simplify the expression?