In the solution to this system, what is the value of z?

$\displaystyle-{3}{x}+{y}+{z}=-{1}\usepackage{cmbright}\Huge\$

$\displaystyle{2}{x}+{y}-{z}=-{4}\usepackage{cmbright}\Huge\$

$\displaystyle{x}-{y}+{z}={7}\usepackage{cmbright}\Huge\$

$\displaystyle{z}=\usepackage{cmbright}\Huge\$ _____