What is the sum of the two solutions to x² - 3x - 40 = 0?

Factor: $x^2 - 3x - 40 = (x - 8)(x + 5)$, since $-8 \cdot 5 = -40$ and $-8 + 5 = -3$. The roots are $x = 8$ and $x = -5$, and they sum to $8 + (-5) = 3$. By Vieta's formulas, the sum of the roots of $x^2 + bx + c$ is $-b = 3$. A common wrong answer is $-3$, which forgets to flip the sign of the $x$-coefficient. Another is $-40$, the product of the roots. A third is 13, which comes from adding the absolute values $8 + 5$. The sum is the opposite of the middle coefficient.