Correct. Add 49 to both sides of the equation: $\displaystyle{{x}}^{{2}}={49}\usepackage{cmbright}\Huge\$

Use the Square Root Property: $\displaystyle{x}={7}\usepackage{cmbright}\Huge\$ or $\displaystyle{x}=-{7}\usepackage{cmbright}\Huge\$

The missing number is 7.