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Remember that in order to add 3x and 6x together, I simply added their
coefficients and then multiplied by the variable they both contain to get 9x,
since 3 plus 6 is 9, and the variable in both terms is x. We can do the same thing
to deal with the y terms over here. So negative 5y plus 9y is going to be equal
to whatever negative 5 plus 9 is times y. We know that negative 5 plus 9 is equal to
4. So our answer is 4y. And we can fill that in the box. Over here in our original
expression, we have four different terms. And over here, we only have two terms.
This is why we call this simplifying expressions. This expression is way more
simple than the one over here is. Remember that I was allowed to put an equal sign
here, between these two expressions, because they are exactly mathematically
equivalent. In order to come up with the expression over here, I only played with
the terms inside of this expression. I didn't bring anything in from the outside.
The only way I modified it was by squishing together things that were
already there.