TOPIC: PERT PMP Question

Can somebody please provide explanation to the following PERT Question from Oliver Lehmann's free sample questions:
Together with your team, you applied three-point estimation on a critical path which consists of two activities.
The following duration uncertainties are all calculated assuming a ±3sigma confidence interval.
The duration uncertainty—defined as pessimistic minus optimistic estimate—of the first activity is 18 days; the second estimate has an uncertainty of 24 days. Applying the PERT formula for paths, what is the duration uncertainty of the entire path? 94
o 21 days
o 30 days
o 42 days
o No statement is possible from the information given.

The correct answer is 21 days, but I am not sure how arrive at this answer. Please advise.

First P-O = 18
Second P-O = 24
2P-2O = 42
So, P-O = 21

Can somebody explain this a little bit more? Still not quite understand.