6 goes exactly into 18 and 24, 3 and 4 times respectively. So -6 goes into these numbers -3 and -4 times. So we can reduce the expression after dividing by -6 to -3*2+4y. The minus sign in front of y has changed to plus because two minuses make a plus in division and multiplication as well as in addition and subtraction. Or you can look at it this way: 4*-6=-24 and 3*-6=-18. Think of minus as owing money. -6 could mean I owe someone 6 pence. If I owe someone 4 times as much as this, I'll owe them 24 pence. Still negative at -24.