Question

factory completely 16x2-81

Answer 1

$16x^2-81=(4x-9)(4x+9)$

Answer 2

This is a difference of squares

presuming you mean

16x² - 81

16 is a square of 4

x² is a square of x

81 is a sqaure of 9

and since they are subtracted from each other its a difference of squares

so its

(4x + 9) (4x - 9)