While correctly adding exponents to simplify $\displaystyle{{y}}^{{2}}{{y}}^{{3}}\usepackage{cmbright}\Huge\$ to $\displaystyle{{y}}^{{5}}\usepackage{cmbright}\Huge\$ you may have mistakenly added exponents to simplify $\displaystyle{{\left({{y}}^{{2}}\right)}}^{{5}}\usepackage{cmbright}\Huge\$ . The power rule tells you to multiply these exponents.