The statement is false. The function $\displaystyle{y}={3}{x}-{4}\usepackage{cmbright}\Huge\$ is not a proportional function. When $\displaystyle{x}={1},{y}=-{1},\usepackage{cmbright}\Huge\$ which equals -1 times 1, but when $\displaystyle{x}={2},{y}={2},\usepackage{cmbright}\Huge\$ which equals 1 times 2.