It is given that

$$\begin{array}{}\text{(1)}& y=2x+6\end{array}$$

$$\begin{array}{}\text{(2)}& 6x+12=3y\end{array}$$

Step 1: Plug (1) into (2)

$$\begin{array}{}\text{(3)}& 6x+12=3y=3(2x+6)\end{array}$$

Step 2: Simplify (3)

$$6x+12=6x+18$$

$$=>12=8$$

Step 3: Notice that we have come to an impossible conclusion, so there’s no solution of x and y that will make 12 equal to 8.