Consider the following system of equations.

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

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

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

Which value of $\displaystyle{a}\usepackage{cmbright}\Huge\$ will result in a system that has infinitely many solutions?