Correct. Add 7 to both sides: $\displaystyle{2}{{x}}^{{2}}={128}\usepackage{cmbright}\Huge\$

Divide both sides by 2: $\displaystyle{{x}}^{{2}}={64}\usepackage{cmbright}\Huge\$

Use the Square Root Property: $\displaystyle{x}={8}\usepackage{cmbright}\Huge\$ or $\displaystyle{x}=-{8}\usepackage{cmbright}\Huge\$

The missing number is 8.