Question

What's...factorise: 25x²-y²?

Answer 1

Factorising an equation is putting it into bracket form.
This has square numbers and so needs double brackets

(5x - y) (5x+y)
If you then check this by multiplying the brackets out again you will get:
 5x^2 - 5xy + 5xy - y^2

The positive and negative 5xy's cancel each other out and you are left with its original form. Your answer is
(5x - y) (5x+y)