You may have chosen this answer because all three values add up to 19. However, to be correct, the values need to make each of the conditions true. The correct system to start with is $\displaystyle{x}+{y}+{z}={19}\usepackage{cmbright}\Huge\$ , $\displaystyle{y}={2}{x}\usepackage{cmbright}\Huge\$ , and $\displaystyle{4}{x}-{z}={2}\usepackage{cmbright}\Huge\$ .