Question

factor 16^2-81

Answer 1

Is there a variable somewhere?

Answer 2

16x^2-81

Answer 3

Ok, notice you have a difference.
Also notice you have 16x^2 which is the square of 4x and 81 which is the square of 9.

Answer 4

This is the factoring of the difference of two squares.
in general, it factors like this:
a^2 - b^2 = (a + b)(a - b)