Subtraction looks at the difference between two numbers. In the chapter Moving Up The Number Line, we discussed how to read equations (number sentences). The first number in an equation tells us the starting point on our number line. The operator (something like + or -) and the next number tell us how many places to move along the number line. When we are subtracting integers, we move to the left along the number line. Let's look at a number line to explain this.
Think: This means "start at $5$5 on the number line and move back (to the left) $4$4 spaces."
It's just the same when you're subtracting negative numbers. Let's look at another example.
Think: This means "start at $-3$−3 on the number line and move back (to the left) $6$6 spaces."
The same rules apply for addition and subtraction of integers, even if we have more than $2$2 numbers.
Play around with the applet below to practice addition and subtraction of positive and negative numbers.