Why is parentheses x + 9 parentheses x - 9 parentheses not equivalent to 2 x ^ 2 - 18

(x + 9) (x - 9)

follow the FOIL method

first

x(x) = x^2

outside 
x(-9) = -9x

inside
9(x) = 9x

last 
9(-9) = -81

 x² - 9x + 9x - 81
simplify: x² - 81

x² - 81 is your answer

hope this helps

it is not - 18, because you are multiplying, not adding, and even if you are adding, the answer wouldn't have a -18

hope this helps