[tex]16x^2-81=(4x-9)(4x+9)[/tex]
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)
