-7 + 3 + 10 = -4 + 10 = 10 - 4 = 6.
To turn 13 into 10, you need to break it up into 10 + 3, which does not change the value of 13. Then, you add 3 to -7, which results in -4. Next, you add -4 to 10 (or rather, subtract 4 from 10), which results in 6.
