You probably substituted correctly into Equation A, but then made a mistake solving for x. If you check your answer by substituting into Equation A, you get $\displaystyle{3}{\left({13}\frac{{2}}{{3}}\right)}+{2}{\left({5}\right)}={31}\usepackage{cmbright}\Huge\$ or $\displaystyle{41}+{10}={31}\usepackage{cmbright}\Huge\$ , which is false.