This one is not correct. You used the distributive property wrong. Using the FOIL method, 2x times -3 is -6x, while 2x times 3 is 6x. Since one is 6x and one is -6x, these terms cancel out. Therefore, the correct answer should be 4x^2 - 9.
You could also get this from doing (a+b)(a-b) = a^2 - b^2, but I wouldn't worry about that now.