Substitute $\displaystyle{8}-{2}{y}\usepackage{cmbright}\Huge\$ for x in the first equation: $\displaystyle{7}{y}-{5}{\left({8}-{2}{y}\right)}={45}\usepackage{cmbright}\Huge\$

Simplify: $\displaystyle{7}{y}-{40}+{10}{y}={45}\usepackage{cmbright}\Huge\$ or $\displaystyle{17}{y}-{40}={45}\usepackage{cmbright}\Huge\$

Solve for y: $\displaystyle{17}{y}={85}\usepackage{cmbright}\Huge\$ , so $\displaystyle{y}={5}\usepackage{cmbright}\Huge\$

Substitute this value for y back into the second equation: $\displaystyle{x}={8}-{2}{\left({5}\right)}={8}-{10}=-{2}\usepackage{cmbright}\Huge\$