Answer
$(x-4)(x-7)$
Work Step by Step
To factor a trinomial in the form $x^2+bx+c$, we must find two numbers whose product is $c$ and whose sum is $b$. We then insert these two numbers into the blanks of the factors $(x+\_)(x+\_)$.
In the case of $x^2-11x+28$, we are looking for two numbers whose product is $28$ and whose sum is $-11$. The numbers $-7$ and $-4$ meet these criteria, because $$-4\times-7=28\;\text{and}\;-4+-7=-11.$$When we insert these numbers into the blanks, we arrive at the factors $(x-4)(x-7)$.
