Correct. Add 36 to both sides of the equation: $\displaystyle{{x}}^{{2}}={36}\usepackage{cmbright}\Huge\$

Use the Square Root Property: $\displaystyle{x}={6}\usepackage{cmbright}\Huge\$ or $\displaystyle{x}=-{6}\usepackage{cmbright}\Huge\$

The missing number is 6.