Incorrect. Try solving this system:

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

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

for the variable n. The easiest way to do this is to substitute $\displaystyle{e}={6}{n}\usepackage{cmbright}\Huge\$ into $\displaystyle{139.50}-{e}={7.50}\cdot{\left({21}-{n}\right)}\usepackage{cmbright}\Huge\$ , resulting in $\displaystyle{139.50}-{\left({6}{n}\right)}={7.50}\cdot{\left({21}-{n}\right)}\usepackage{cmbright}\Huge\$ . Then solve for n.