The function f is defined by f(x) = (x-3)(x+7). What is the sum of the zeros of f?

The zeros of $f(x) = (x-3)(x+7)$ are the inputs that make the function equal 0. Setting each factor to zero gives $x = 3$ and $x = -7$. Their sum is $3 + (-7) = -4$, choice D. As a check, expand: $f(x) = x^2 + 4x - 21$, and the sum of the roots is $-\frac{4}{1} = -4$. Choice A (4) comes from reading the factors as $x = -3$ and $x = 7$ with the signs flipped, or from using the coefficient 4 without the negative. Choice B ($-21$) is the product of the zeros. Choice C (10) is the distance between them, $3 - (-7)$.