16x² - 81
This equation is in difference of square form.
we know that if it is difference of square then our answer should look like
( x+y) ( x-y)
According To Question
16x² - 81
(4x - 9) { because 4 *4 = 16 and 9 * 9 = 81 }
(4x - 9) ( 4x + 9) is our answer
Hope this helps.. :)