You probably thought that you had to find the opposite of the second 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{9}{x}-{2}{y}={9}{\left({4}\right)}-{2}{\left({5}\right)}={36}-{10}={26}\usepackage{cmbright}\Huge\$ , not 46.