Another one i'm stuck with, the examples don't show how to calculate for Za/2 a number just appears in the examples and it's not helpful:
Construct a confidence interval for p 1 minus p 2 p1-p2 at the given level of confidence.
x 1 equals x1=387, n1=518 518?, x2=414, n2=598 99?% confidence 99?% confidence interval for p1 and p2