You may have simplified $\displaystyle{{\left({3}-{6}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ incorrectly and gotten $\displaystyle-{9}\usepackage{cmbright}\Huge\$ , instead of 9. Operations inside parentheses come before exponents. So if you have $\displaystyle{{\left(-{3}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ , you are squaring a negative number and will get a positive result.