Answer:
C
Explanation:
For this, you want to treat i like any other variable, and combine like terms. However you need to keep in mind that there is a negative sign before the second set of parentheses. This means everything inside it should have a negative before it. So we can write it like this:
(7 + 3i) - (3 - 9i)
7 + 3i -3 +9i
4 + 12i
Hope that helps!