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?

5 bears!

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: