Factor as the product of two binomials.

X^2 - 10x + 21

You can use the butterfly method. Two numbers that equal A times C must also add up to B. A=1 B=-10 C=21. A times C is 21 and B is - 10. Two numbers that multiply to 21 and add to - 10 are - 7 and - 3. Factored form: (x - 7) (x - 3).