You may have correctly added the equations and gotten $\displaystyle{y}={4}\usepackage{cmbright}\Huge\$ , but then mixed up the variables and substituted this value for x in the first equation. This ordered pair is a solution for the first equation, but not for the second equation. If you substituted into the left side of the second equation you would get $\displaystyle{2}{x}+{9}{y}={2}{\left({4}\right)}+{9}{\left({0}\right)}={8}+{0}={8}\usepackage{cmbright}\Huge\$ , not 48.