It appears that you added all of the exponents. To simplify $\displaystyle{{\left({{z}}^{{6}}\right)}}^{{1}}\usepackage{cmbright}\Huge\$ you need to use the power rule. This tells you to multiply the exponents. Then to multiply by $\displaystyle{{z}}^{{2}}\usepackage{cmbright}\Huge\$ you would use the product rule, in which case the exponents get added. The correct answer is $\displaystyle-{{z}}^{{8}}\usepackage{cmbright}\Huge\$ .