Answer:
10x . . . . . (Note the sign of the middle term is negative. 10x goes in the box.)
Explanation:
The sum of the two roots is ...
... (5 -3i) +(5 +3i) = 10
In a quadratic with leading coefficient 1, the coefficient of the 1st-degree term is the opposite of the sum of the roots. Here, that means the middle term is -10x. The minus sign is given, so the answer is 10x.
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How Do We Know?
When "a" and "b" are roots of a quadratic in x, it has factors (x -a)(x -b). The product of those two factors is ...
... (x -a)(x -b) = x² -(a+b)x +ab
Here, that means the product of the factors (5 -3i)(5 +3i) is ab = 34, which it is. Their sum is (a+b) = 10, so the x-term is -(a+b)x = -10x.