The function f is defined by f(x) = (x+5)(x-9). What is the sum of the zeros of f?

The zeros of $f(x) = (x + 5)(x - 9)$ are the values that make a factor zero: $x = -5$ and $x = 9$. Their sum is $-5 + 9 = 4$. Check by expanding: $f(x) = x^2 - 4x - 45$, and the sum of the roots is $-\frac{-4}{1} = 4$. A common wrong answer is $-4$, which comes from adding the numbers in the factors, $5 + (-9)$, instead of the zeros, whose signs are opposite. Another is $-45$, which is the product of the zeros. Read zeros from the factors with the opposite sign.