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

$\displaystyle{4}{a}+{b}=-{7}\usepackage{cmbright}\Huge\$

$\displaystyle{a}-{2}{c}={5}\usepackage{cmbright}\Huge\$

$\displaystyle{3}{b}+{2}{c}={7}\usepackage{cmbright}\Huge\$

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