you'll want to group them where they'll factor out easier; 3x^2 and 3x match well, and -2xy and -2y match well.
(3x^2 + 3x)(-2xy - 2y)
then you factor out a piece from each:
3x can come out of the first pair
-2y can come out of the second
3x(x + 1) - 2y(x + 1)
the goal of facoring by grouping is to factor in a way that gets you the same binomial within your parentheses, because once those are the same, you can factor the ENTIRE binomial out to leave you with your other factor:
(x + 1)(3x - 2y)
those are your two factors