Explanation
First, think of these numbers on a number line. -8.79 is to the left of 0, and 12.345 is to the right of 0. When you add them together, you're moving 8.79 units to the left from 12.345. This brings you to a point on the number line that is 3.555 units to the left of 0.
Mathematically, you can calculate this by subtracting the absolute value of -8.79 from 12.345: 12.345 - 8.79 = 3.555.
So, when you add -8.79 and 12.345, you get a result of 3.555.