## Introductory Algebra for College Students (7th Edition)

$$-8 + (-13) = -21$$
In this problem, we are adding a $-13$ to an already negative number, $-8$, meaning that we are already on the negative side of the number line and we are going more left on the number line. If we are at $-8$ on the number line and go left $13$ spots, then we will be at $-21$. Therefore, $$-8 + (-13) = -21$$