I'm going back to college after 5 years and am having a little trouble with geometry, one that i haven't been able to find an answer for is split into 3 parts;

A) What is the diagonal of a square who's side is A?

B) What is the side of a square whose diagonal is A?

C) Find the area of an equilateral triangle whose side is A.

I got the first one, but haven't been able to get the second ones. I assume I'm using the Pythagorean Theorem, but haven't been able to get it. If you could write out the steps it would help a lot. Thank you.