Objective:
Obtain an score of 0 after add all the scores in all the rounds
Info given:
Round 1: -9
Round 2: 3
Round 3: -8
Round 4: 2
And we want to estimate the score in round 5 in order to obtain a 0 so we can do this:

Where x represent the score required for round 5 and solving we got:

So then the score required for round 5 is 12