Appendix A - Review - A.4 Factoring Polynomials - A.4 Assess Your Understanding - Page A40: 81

$(x-3)(x-7)$

To factor the second-degree polynomial $x^2 - 10x + 21$, we have to find integers whose product is $21$ and whose sum is $-10$. That is, we have $-3$ and $-7$ where $-3-7=-10$ and $-3\times (-7)=21$. Thus $$ x^2 - 10x + 21=(x-3)(x-7) .$$