Answer:
Difference of two squares factors in this way: a2 - b2 = (a + b)(a -b)
You can determine if it's a difference of two squares, by seeing if both terms are perfect squares, and can be written in the form a2 - b2. Michael already showed how to factor it.
eg. 4x2 - 25 can be written as (2x)2 - 52
now notice we have it in that form a2 - b2 where a = 2x and b = 5.
(2x)2 - 52 = (2x - 5)(2x+5)
Another example
9x2 - 16
This can be written as (3x)2 - 42
Now try factoring it with the difference of two squares.
