You probably thought that you had to find the opposite of the first equation before adding the equations. This ordered pair is a solution for the second equation, but not for the first equation. If you substituted into the left side you would get $\displaystyle{11}{x}+{4}{y}={11}{\left(-{5}\right)}+{4}{\left(-{21}\right)}=-{55}-{84}=-{139}\usepackage{cmbright}\Huge\$ , not 29.