Simplifying and volume calculating

Simplify $\displaystyle \frac{27x}{3xy^2}$ I got 9 somehow....

Simplify $\displaystyle \frac{x-5}{x^2-25}$ I'm just stuck on how to deal with the numbers (-5 and -25)

Simplify $\displaystyle (\sqrt{5}-\sqrt{3})(\sqrt{5}+\sqrt{3})$ I got 2

Total surface area of cylinder X is 25cm^2

Total surface area of cylinder Y is 225cm^2

Height of cylinder X is 5cm

Calculate height of cylinder Y

Volume of Cylinder X is 27cm^3

Calculate volume of Cylinder Y

Again... I have no idea for the method of this volume and height calculating.