Question 889831
Given is x^2 - 4 = 0

      -> x^2 - 2^2 = 0  ( 4 = 2^2 )

      -> (x-2).(x+2) = 0 ( x^2 - a^2 = (x-a).(x+a) )

      -> x-2 = 0  or   x+2 = 0

      -> x = 2   or   x = -2