Question

simplify this expression (-4-9i)+(5-7i)

Answer 1

To simplify you combine like terms. 
You open parenthesis-
-4 - 9i + 5 - 7i
combine like terms-
1 - 16i   is your answer. 
Hope this helps!

Answer 2

You can simplify the real and imaginary parts independently:

-4+5 = 1
-9i-7i = -16i

So the simplified expression is 1-16i