Answer:
1st blank = 12
2nd blank = 3
3rd blank = (-12)
4th blank = (-21)
Explanation:
for the first problem, just remeber that "minus a negitive, equals plus a positive" then -9 + 12, equals a positive 3.
and for the 2nd problem, when there is no negitive on the 2nd term, just remeber that if you change the sign in the middle, change the sign that follows. so that -9 - 12 then turns into -9 + (-12).
hope this helped!