One method is called the ac method:-
For example factor
3x^2 + 2x - 21
ac ( first coefficient times the last) = 3*21 = -63
now we need to find 2 factors of -63 which add up to the coefficient of x ( + 2):-
+9 and - 7 look good so we can write the original expression as:-
3x^2 + 9x - 7x - 21
Factor by grouping:-
= 3x(x +3) - 7(x + 3) ( x + 3) is common so:-
= (3x - 7)(x + 3) Answer
