Jeannette is correct in this situation. You cannot subtract a negative number from anything. If you ever end up having two negative signs in a row, you must revert them to a positive (add). In this case:
-10 - (-8) = -10 + 8
When adding a positive to a negative, you can use the commutative property to know that this is the same as simply subtracting 10 from the positive integer 8:
10 - 8 = -2
If you need further explanation, let me know.
Hope this helps!
~ArchimedesEleven