Since $\displaystyle{y}={k}{x},\usepackage{cmbright}\Huge\$ we know that $\displaystyle{k}\usepackage{cmbright}\Huge\$ must be - 5, since when $\displaystyle{x}={1},{y}=-{5},\usepackage{cmbright}\Huge\$ which equals -5 times 1. Therefore, when $\displaystyle{x}={4},{y}\usepackage{cmbright}\Huge\$ must equal $\displaystyle-{5}\usepackage{cmbright}\Huge\$ times $\displaystyle{4}\usepackage{cmbright}\Huge\$ , which is $\displaystyle-{20}.\usepackage{cmbright}\Huge\$