Answer: 12
Step-by-step explanation:
I don't know if you mean using the Complements Method but I really wouldn't recommend using this method for all problems unless your teacher asked you to specifically.
Here is how to:
1. take the "complement" of the number we are subtracting (The "complement" is the number to add to make 10 (or 100, 1000, etc, depending on how many digits we have)
2. add it to to the number we are subtracting from
3. discard the extra "1" on the left
If you mean to do it mentally, think about it like this:
20 + (-9) = 11 You are essentially adding negative nine to twenty, not subtracting nine. If you want to see this better, try using a number line. When you add a negative number, you move to the left on the number line.
Same thing with 8 + (-7) = 1
You are adding negative seven to eight.
All that is left to do is adding both of the results together, and you get twelve.
11 + 1 = 12
is 
, since the original number is positive
is 
, since the original number is negative