Correct. The information gives you two equations:

(A) $\displaystyle{y}={x}-{14}\usepackage{cmbright}\Huge\$ (B) $\displaystyle\frac{{{x}+{y}}}{{2}}={84}\usepackage{cmbright}\Huge\$

Substitute $\displaystyle{x}-{14}\usepackage{cmbright}\Huge\$ for y in (B): $\displaystyle\frac{{{x}+{x}-{14}}}{{2}}={84}\usepackage{cmbright}\Huge\$

Simplify and multiply both sides by 2: $\displaystyle{2}{x}-{14}={168}\usepackage{cmbright}\Huge\$

Add 14 and divide by 2: $\displaystyle{x}={91}\usepackage{cmbright}\Huge\$ , so $\displaystyle{y}={91}-{14}={77}\usepackage{cmbright}\Huge\$