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Brian bought 3 packs of mints with a number of mints in each pack.

write an expression.

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So your looking for the way to write that. We do not have exact numbers so we need a placeholder. In Algebra we use letters to hold a spot for values we do not know the answer too yet.

With me so far? When we write that in math terms, we have to explain what each placeholder means. You might see phrases like, "Let X be..." You need to start with an explanation. In this case we are after a way to explain a relationship between packages or groups of mints and their sum total. So we are talking about # of mints. We can let X be the total number of mints. 

You have 3 packages of them right? How would you write that? 

The next part is your total number of mints in all 3 packages, right? We don't know what that number is so, we can replace it with a "placeholder". X is a placeholder but we are using that one. Pick a new placeholder. We have to explain what we are using as our placeholder. Let Y be total number of mints. So you now have two different pieces - you have ?X=? .

Fill in the blanks. Here is another example, you can try. Lets say I want to write an expression for how many female friends I have on Facebook. I know I have a total of 200 friends, and 18 of them are male. How would I write that in math?