If the function is proportional, then $\displaystyle{y}={k}{x}\usepackage{cmbright}\Huge\$ for some constant x. When $\displaystyle{x}={3},{15}={k}\cdot{3}\usepackage{cmbright}\Huge\$ , so $\displaystyle{k}\usepackage{cmbright}\Huge\$ must equal $\displaystyle{5}.\usepackage{cmbright}\Huge\$ Therefore, when $\displaystyle{x}={2},{y}={5}\cdot{2}={10}.\usepackage{cmbright}\Huge\$