the difference between 7 times a number and 9 less than a number

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the difference between 7 times a number and 9 less than a number

The phrase a number means an arbitrary variable, let's call it x.

7 times a number means we multiply x by 7
7x

9 less than a number means we subtract 9 from x
x - 9

The difference between the two expressions means we subtract (x - 9) from 7x
7x - (x - 9)

Simplifying this, we have:
7x - x + 9

Grouping like terms, we get:
6x + 9