Correct. Add the first and second equations: $\displaystyle{2}{d}+{5}{e}={16}\usepackage{cmbright}\Huge\$

Add the first and third equations: $\displaystyle-{2}{d}+{4}{e}={20}\usepackage{cmbright}\Huge\$

Add these two new equations: $\displaystyle{9}{e}={36}\usepackage{cmbright}\Huge\$ , so $\displaystyle{e}={4}\usepackage{cmbright}\Huge\$

Substitute this into the first new equation: $\displaystyle{2}{d}+{20}={16}\usepackage{cmbright}\Huge\$ , so $\displaystyle{d}=-{2}\usepackage{cmbright}\Huge\$

Substitute d and e into the first original equation: $\displaystyle-{10}+{12}+{2}{f{=}}{8}\usepackage{cmbright}\Huge\$ , so $\displaystyle{f{=}}{3}\usepackage{cmbright}\Huge\$

Check the values of d, e, and f in all three original equations.