You may have correctly added the equations and gotten $\displaystyle{x}={3}\usepackage{cmbright}\Huge\$ , but then mixed up the variables and substituted this value for y in the second equation. This ordered pair is a solution for the second equation, but not for the first equation. If you substituted into the left side of the first equation you would get $\displaystyle{5}{x}+{4}{y}={5}{\left({7}\right)}+{4}{\left({3}\right)}={35}+{12}={47}\usepackage{cmbright}\Huge\$ , not 23.