You're standing there at the edge of the answer but you're afraid to jump in. Take a long hard look inside the parentheses. Stare at it and don't blink.
Isn't 4n^2. a square ? Isn't. 9. a square ? Is that so ! Then isn't (4n^2 - 9) the difference of squares ! ? ! Go ahead. Take the plunge.
meters, which rounds to roughly 636cm.