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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?

3 bears + 2 bears = 5 bears

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?

( 1 bear + 4 cats ) + ( 6 bears + 2 cats + 3 dogs ) = 7 bears + 6 cats + 3 dogs

7 bears, 6 cats and 3 really stupid looking dogs.

Get it?  Now we can do this:

( b + 4c ) + ( 6b + 2c + 3d ) = 7b + 6c + 3d