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now let's think about expressions with more than one variable so say I had the expression a plus a plus I'll do a really simple one a plus B and I want to evaluate this expression when a is equal to seven and B is equal to two and I encourage you to pause this and try this on your own well we wherever we see the a we would just replace it with the 7 and wherever we were C we see the B we'd replace it with the 2 so when I equals seven and B equals two this expression will be seven plus two which of course is equal to nine so this expression would be equal to nine in this circumstance let's do a slightly more complicated one let's say we have the expression x times y minus y minus y plus X plus actually let's make it plus three X or another way of saying it plus three times X so let's evaluate this when x is equal to when X is equal to 3 and Y is equal to two and once again I encourage you to pause this video and try this on your own well everywhere we see an X let's replace it with a 3 everyplace we see a Y let's replace it with a 2 so this is going to be equal to 3 times y and Y is 2 in this case 3 times 2 minus 2 plus plus this 3 times X but X is also now equal to 3 so what is this going to be equal to well this is going to be equal to 3 times 2 is 6 this 3 times 3 is 9 so it simplifies to 6 minus 2 which is 4 plus 9 which is equal to 13 so in this case it is equal to 13