If the function is proportional, then $\displaystyle{y}={k}{x}\usepackage{cmbright}\Huge\$ for some constant $\displaystyle{x}.\usepackage{cmbright}\Huge\$ When $\displaystyle{x}={1},{4}={k}\cdot{1},\usepackage{cmbright}\Huge\$ so $\displaystyle{k}\usepackage{cmbright}\Huge\$ must be equal to 4. Therefore, when $\displaystyle{x}={2},{y}={4}\cdot{2}={8}.\usepackage{cmbright}\Huge\$