(x + 6)(x - 6)
x2-81 = (x-9)(x+9)
x^2 - y^2 - 4 is in its simplest form.
The expression x2 + 1x - 12 can be factored out as (x - 3)(x + 4)
x³ - x² + x - 3 (x² + 1)(x - 1) - 2
(x - 2)(2x - 3)
(x - 12)(x + 12)
(x - 3)(x + 3)
The value of that expression depends on the value of "x". You can't really simplify it, except that you can factor it by separating the common factor "x".
x2 - 9x + 20 = (x - 4)( x - 5)
x(x-1)
(x + 2)(x - 4)