+(-24) is the same as -24. Adding a negative is the same as subtracting a positive of the same magnitude.
Now the problem becomes: -5+20-24.
Having problems with negatives? Negatives are steps backward, positives are steps forward: mark your starting point; take 5 steps back and 20 steps forward, then 24 steps back. You're 9 steps back from where you started. That's -9.