Answer:
(x +7)(x -3)
Step-by-step explanation:
The constants in the binomial factors must multiply to give -21 and must add to give +4. Since you're familiar with your multiplication tables, you know that ...
-21 = -1·21 = -3·7 = -7·3 = -21·1
The sums of these factor pairs are 20, 4, -4, -20. Clearly the pair of choice is -3 and 7. Then the binomial factors are ...
(x -3)(x +7)
