Substitute $\displaystyle{y}={5}\usepackage{cmbright}\Huge\$ into Equation A: $\displaystyle{3}{x}+{2}{\left({5}\right)}={31}\usepackage{cmbright}\Huge\$

Simplify: $\displaystyle{3}{x}+{10}={31}\usepackage{cmbright}\Huge\$

Solve the equation for x: $\displaystyle{3}{x}={21}\usepackage{cmbright}\Huge\$ , so $\displaystyle{x}={7}\usepackage{cmbright}\Huge\$