While correctly multiplying exponents to simplify $\displaystyle{{\left({{y}}^{{2}}\right)}}^{{4}}\usepackage{cmbright}\Huge\$ to $\displaystyle{{y}}^{{8}}\usepackage{cmbright}\Huge\$ you may have mistakenly multiplied exponents to simplify $\displaystyle{{y}}^{{3}}{{y}}^{{4}}\usepackage{cmbright}\Huge\$ . The product rule tells you to add these exponents.