Jack - math

I have solved this equation on accident, but my issue is that I cannot figure out why: y^2 - 9 is factored as (y−3)(y+3) instead of (y+3)(y-3) What makes these type of answers one way or the other?

With an equation like: (y^7)^-2, why is the answer = 1/(y^14) and then I look at a similar type of problem: (x^-3)^-5 = x^15. How come one answer needs to have a numerator and denominator while the other is just simple like x^15?

I keep answering similar questions incorrectly because I cant figure out which + or - sign goes where.