It is given that

$$ y=2x+6 \tag{1} $$

$$ 6x+12=3y \tag{2} $$

Step 1: Plug (1) into (2)

$$ 6x+12=3y =3(2x+6)\tag{3} $$

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.