Theory
Negative numbers are less than zero. They are used for temperatures below zero, bank balances, lifts below ground floor, and differences.
On a number line, moving right makes the number larger and moving left makes the number smaller.
Adding a positive number moves right. Subtracting a positive number moves left.
Adding a negative number has the same effect as subtracting. For example, 6 + -4 = 2.
Subtracting a negative number moves right because you are taking away a negative. For example, 6 - -4 = 10.
For multiplication and division, same signs give a positive answer. Different signs give a negative answer.