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Now, let's think about expressions with more than one variable. So, let's say I had the expression "a + b" and I were to ask you what is this expression equal to when "a" is equal to 5 and "b" is equal to 7 Well, in this situation, this expression would be... instead of an "a" we would have a 5 it would be 5 plus... instead of a "b", we would have a 7. So it would be 5 + 7 is equal to 12. Now let's try another expression. Let's say we had the expression "10x - 3y" And, let's think about what would this expression evaluate to when "x" is equal to - 1 and when "y" is equal to 5. Well, let's just evaluate it. We have 10 times... instead of an "x", we know now that "x" is equal to -1, it would be 10 (-1) - 3 (5). So three times... instead of a "y", we will put a 5 there. - 3 (5). And so 10 (-1), that is -10 and we're going to subtract from that 3 (5). You want to do the multiplication first by order of operations that's the convention. 3 times 5 is 15. So, it is -10 - 15, which is equal to -25. And now, you have the next exercise to practice some of these a little bit more.