What impact does negative rational numbers have on whether you add or subtract rational numbers?

Subtracting a rational number is the same as adding it's opposite. If you have a double negative, you add it like it is a positive, than keep the answer you get. Examples:

1) 6 3/4 - (-8 4/8) is the same as saying 6 3/4 : 8 4/8.

2) - 38 5/8 - 6 5/12 translates to - 38 5/8 + (-6 5/12)