The trick to this is to add or subtract things that look alike. It's usually called "combining like terms."
Here's a really basic example:
If you have 3 bears and someone gives you 2 more bears, how many bears do you have?
Now, what if you have one bear and 4 cats and someone gives you 6 bears, 2 cats and 3 dogs? What do you have?
7 bears, 6 cats and 3 really stupid looking dogs.
Get it? Now we can do this: