Solve the first equation for x: $\displaystyle{x}={16}-{y}\usepackage{cmbright}\Huge\$

Substitute $\displaystyle{16}-{y}\usepackage{cmbright}\Huge\$ for x in the second equation: $\displaystyle{7}{\left({16}-{y}\right)}-{6}{y}=-{31}\usepackage{cmbright}\Huge\$

Simplify: $\displaystyle{112}-{7}{y}-{6}{y}=-{31}\usepackage{cmbright}\Huge\$ or $\displaystyle{112}-{13}{y}=-{31}\usepackage{cmbright}\Huge\$

Solve for y: $\displaystyle-{13}{y}=-{143}\usepackage{cmbright}\Huge\$ , so $\displaystyle{y}={11}\usepackage{cmbright}\Huge\$

Substitute this value for y into the equation you started with: $\displaystyle{x}={16}-{11}={5}\usepackage{cmbright}\Huge\$