To factor a polynomial such as x^3-9x^2+11x+21, it is helpful to know that the product of (x-a)(x-b)(x-c) starts out x^3-(a+b+c)x^2 so the sum of the roots is 9, and the last term will be -abc so the product of the roots is -21. Since the factors of 21 are 1,3,7 the combination of -1,3,7 is the one ...