Add the equations to get: $\displaystyle{5}{y}={25}\usepackage{cmbright}\Huge\$

Solve for y to get $\displaystyle{y}={5}\usepackage{cmbright}\Huge\$ and substitute into the first equation:

$\displaystyle{5}{x}+{2}{\left({5}\right)}={5}\usepackage{cmbright}\Huge\$ , or $\displaystyle{5}{x}+{10}={5}\usepackage{cmbright}\Huge\$ , so $\displaystyle{5}{x}=-{5}\usepackage{cmbright}\Huge\$ and $\displaystyle{x}=-{1}\usepackage{cmbright}\Huge\$