Question

Factorise y^2 - 16 (y squared)

Answer 1

$y^2-16=y^2-4^2=(y-4)(y+4)\\\\\text{used}\ a^2-b^2=(a-b)(a+b)$

Answer 2

Sqrt(y^2) = y
sqrt(16) = 4
-4y + 4y = 0
The answer is
(y-4)(y+4)