Substitute y=x+3 in the second equation: x+3=2x+12; subtract 3 from each side: x=2x+9; subtract 2x from each side: -x=9 so x=-9. Substitute this value of x into y=x+3, y=-9+3=-6. Could also substitute for x in the other equation: y=2x+12, y=-18+12=-6.