TOPIC: Need help understanding Standard Deviation associated with product variation.

Here is the practice question:
..standard deviation associated with the product variation was 0.7 inches and with the measurement variation was 0.5 inches, What is the total standard deviation here?
a. 1.2
b. 0.86
c. 0 .49
d.0.25
the answer was b . 0.86. I have no idea how to get to this number. I was looking at the standard deviation formula: P - O/ 6 = standard deviation. Answer states to use variation formula [P -O/6 ]2. I cannot tell what the Pessimistic is or the Optimistic is in this question? Can you please help me?

Heather,
Can you please also post the source of the question you posted? It is important to give credit to the original author.
Thanks,
Mark

the source is a practice exam from Project Management Professional Certification Exam Prep - Third Edition.
Will you be able to provide me with some assistance? i'm taking my exam this week. Thank you