4x^2-81 let's solve this.
First, Rewrite it in the form a^2−b^2, where a=2x and b=9
(2x)^2−92
Second, use difference of squares: a^2-b^2=(a+b)(a-b)
(2x+9)(2x-9)
difference of squares==> a^2-b^2=(a+b)(a-b)
For Example: x^2-4
First, Rewrite it in the form a^2−b^2, where a=x and b=2
x^2-2^2
and then again use the difference of squares: a^2-b^2=(a+b)(a-b)
(x+2)(x-2)