Incorrect. You need to add $\displaystyle{{\left(\frac{{-{24}}}{{2}}\right)}}^{{2}}={{\left(-{12}\right)}}^{{2}}={144}\usepackage{cmbright}\Huge\$ to both sides to complete the square: $\displaystyle{{x}}^{{2}}-{24}{x}+{144}={0}\usepackage{cmbright}\Huge\$

Then write the left side as the square of a binomial, take the square root of both sides, and solve for x.