Adding two negative numbers together? Just add the absolute value of each number together, put a negative sign in front, and you have your answer! See how it's done in this tutorial.
In this tutorial you'll see how you can think of absolute value in a very intuitive way. Let us know if you have any questions about it!
Subtracting a positive from a negative? Just remember: subtracting a positive is the same as adding a negative. See how it works in this tutorial!