You may have rewritten the equation as $\displaystyle{{x}}^{{2}}+{10}{x}+{100}={107}\usepackage{cmbright}\Huge\$ , and mistakenly thought that the left side was a perfect square. You should instead add 25 to both sides of $\displaystyle{{x}}^{{2}}+{10}{x}={7}\usepackage{cmbright}\Huge\$ to complete the square, and then factor the left side.