Actually it's not, that would give you 6x^2-3x+8x-4, or 6x^2+5x-4, and you said you wanted 6x^2-5x-4, right?
The factor for that is in my initial response, which is (2x+1)(3x-4). As for the critical points....I did that along time ago, I need to remember how to do them. If I can figure it out I'll let you know.