Try to get it into form (ax+b)(cx+d) a,b,c,d can be negative or positive 3x^2+7x+2 2 and 7 are positive so b and d will be positive as well 3 is prime so we have to have (3x+?)(1x+?) 2 is also prime try (3x+2)(1x+1), expanded it is 3x^2+5x+2, nope (3x+1)(x+2), expanded is 3x^2+7x+2, yep mostly a trial and error sort of thing after some practice, it might get easier for you to see
