Question

How do you do _+-5=-13

Answer 1

X + -5 = -13
x - 5 = -13
    +5    +5

x = -8

Answer: The blank is -8

Answer 2

First make the dash an x. Then move over to the other side of the = so it looks like this:
-5=-13-x
You take -13 to the other side of the equal sign so it looks like this:
-5+13=-x
Then:
8=-x or x=-8
Hopefully that makes sense.