We write out and number the equations
[tex]\begin{gathered} x+y=-3--------------------(1) \\ 4x+4y=-12------------------(2) \\ From\text{ equation (1), we have:} \\ y=-x-3 \\ \text{From equation(2)} \\ 4x+4y=-12 \\ 4y=-12-4x \\ \Rightarrow\frac{4y}{4}=\frac{-12-4x}{4} \\ \Rightarrow y=-x-3 \end{gathered}[/tex]