Substitute $\displaystyle{3}{x}+{1}\usepackage{cmbright}\Huge\$ for y in the first equation: $\displaystyle{12}{x}={4}{\left({3}{x}+{1}\right)}-{4}\usepackage{cmbright}\Huge\$

Simplify: $\displaystyle{12}{x}={12}{x}+{4}-{4}\usepackage{cmbright}\Huge\$ or $\displaystyle{12}{x}={12}{x}\usepackage{cmbright}\Huge\$

Since this equation is true no matter what value x is, this means that every value of x will give you a solution. So there are an infinite number of solutions.