Correct. Because the problem says "directly proportional," the equation to use is $\displaystyle{y}={k}{x}\usepackage{cmbright}\Huge\$ , where y is the cost and x is the number of square feet.

Solve $\displaystyle{120},{000}={k}{\left({1},{500}\right)}\usepackage{cmbright}\Huge\$ for k. This gives you $\displaystyle{k}={80}\usepackage{cmbright}\Huge\$ .

Then for $\displaystyle{x}={2},{400}\usepackage{cmbright}\Huge\$ : $\displaystyle{y}={80}{\left({2},{400}\right)}\usepackage{cmbright}\Huge\$ or $\displaystyle{y}={192},{000}\usepackage{cmbright}\Huge\$