Question

(x - 9) (x + 9)

Answer 1

This is a difference of squares.
=x^2-81

Answer 2

how did you end up subtracting? i know that you multiply 9 x 9 but how do you know whether the answer should be addition or subtraction?

Answer 3

You just foil it out.

\((x-9)(x+9) = (x*x) + (9*x) + (-9*x) + (-9*9) \)
First Outer Inner Last

\( = x^2 + 9x - 9x - 81\)
\(= x^2 - 81\)