Correct. The system:

$\displaystyle{e}={6}{n}\usepackage{cmbright}\Huge\$

$\displaystyle{139.50}-{e}={7.50}\cdot{\left({21}-{n}\right)}\usepackage{cmbright}\Huge\$

can be solved for n, which is 12. That means $\displaystyle{21}-{n}={21}-{12}\usepackage{cmbright}\Huge\$ , which is 9.