Correct. Add the second and third equations: $\displaystyle{a}+{3}{b}={12}\usepackage{cmbright}\Huge\$

Multiply the first equation by $\displaystyle-{3}\usepackage{cmbright}\Huge\$ : $\displaystyle-{12}{a}-{3}{b}={21}\usepackage{cmbright}\Huge\$

Add these equations: $\displaystyle-{11}{a}={33}\usepackage{cmbright}\Huge\$ , so $\displaystyle{a}=-{3}\usepackage{cmbright}\Huge\$

Substitute into the first equation: $\displaystyle-{12}+{b}=-{7}\usepackage{cmbright}\Huge\$ , so $\displaystyle{b}={5}\usepackage{cmbright}\Huge\$

Substitute the value for b into the third equation: $\displaystyle{15}+{2}{c}={7}\usepackage{cmbright}\Huge\$ , so $\displaystyle{c}=-{4}\usepackage{cmbright}\Huge\$

Check your solution in all three equations.