Answer: 1) a perfect fifth has a distance of 7 semitones to our original note, and in this case, 7 semitones to the left.
So if the note marked is the black one, the p5 will be a G#, if is the wite one next to it, the p5 will be an A.
2) To find key signatures, you can count the shields in the signature, here we have five sharp symbols (#), this can be an F sharp in the major key or a D sharp in the minor key, you can find tables about it that are very helpful.