If you solve a system and end up with the equations $\displaystyle{x}={8}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}={8}\usepackage{cmbright}\Huge\$ , then that means the solution is (8, 8). Here, if you substitute $\displaystyle{3}{x}+{2}\usepackage{cmbright}\Huge\$ for y in the second equation and then simplify, you will end up with the equation $\displaystyle{8}={8}\usepackage{cmbright}\Huge\$ , which means something very different. This equation is true regardless of the value of x.