Answer:
Hello,
I have reply too quick in comments (sorry)
Step-by-step explanation:

I believe it’s 12. But I’m not sure
we know that
If two numbers have a sum of zero, then we say they are additive inverses.
so
z+0.8=0 -------> the sum is equal to zero
z=-0.8------> the additive inverse of z is equal to -0.8
using a graph tool
see the attached figure
<u>the answer Part a) and part b) in the attached figure</u>