How do you factor 4-y²?

Answer 1

4 - y2 can be written as 22 - y2. This has become of the form of x2 - y2.

Expansion for x2 - y2 is (x+y)(x-y).

So, 4 - y2 = 22 - y2 = (2+y)(2-y)

Answer 2

(2 - y)(2 + y)