You know that in a number line the numbers go like this
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and so on.
If you take 7 and take away 3 it means you don't have 3 any more.
Think of it like this...
I have 7 cubes. I lost 3 cubes. How many cubes do I have now?
Well you know that 3 backward from 7 is 4, so 7-3 is 4.
