If you are asking the sum of 13 + (-24)
Then, + and - will ultimately become -
So, 13 + (-24) = 13-24 = -11 Ans.
Consider, + as your friend and - as your enemy.
(-)(+) = enemy's friend = your Enemy = -
(-)(-) = enemy's enemy = your friend = +
(+)(+) = friend's friend = your friend = +