Okay, so you need to factor it. Notice how there are only two terms, 4x^2 and 9, and there's no term with just an x. The two terms are then going to be squares. We need the terms with just an x to cancel out.
Square root of 4x^2 is 2x. Square root of 9 is 3.
So (2x+3)(2x-3). To see the other way around, multiply these out, and you can see the middle terms with an x cancel out.
(2x+3)(2x-3) = 4x^2 - 6x + 6x - 9
The blue terms cancel out.
4x^2 - 9 = (2x+3)(2x-3)
Answer is A.
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