You can put this solution on YOUR website! x+3y=12, x=-3y+12 now substitute (-3y+12) for x and solve for y
2(-3y+12)-4y=24
-6y+24-4y=24
-10y=24-24
-10y=0
y=0 answer. now replace y by 0 and solve for x
x+3*y=12
x+0=12
x=12 answer.
proof
2*12-4*0=24
24=24