You are probably thinking of the substitution method. To use that method you could, for example, start by solving the first equation for x. In that case, adding $\displaystyle{2}{y}\usepackage{cmbright}\Huge\$ to both sides of Equation A would be correct. But to use the elimination method, you want to take advantage of having opposite terms, $\displaystyle-{2}{y}\usepackage{cmbright}\Huge\$ and $\displaystyle{2}{y}\usepackage{cmbright}\Huge\$ , in the two equations.