Two's Complement, subtracting two negative binary numbers

Question

So I'm doing CS and the lecturer wrote out a new binary question that I can't wrap my head around.

Basically, how do you subtract two negative numbers if they are in binary?

Specifically how to do it using Two's Constant which was just introduced.

For example: -2 - -7 = 5
Or: 1110 - 1001

I don't understand how you use those bits to get the correct answer, Ie 5.

Can anyone help please?

Answer 1

If you have

  1110 -2
- 1001 -7
------
  0101  5

and this is the same as

  1110 -2
+ 0111  7
------
  0101  5