4 times the difference of 6 times a number and 7

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4 times the difference of 6 times a number and 7

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

6 times a number
6x

The difference of 6x and 7 means we subtract 7 from 6x:
6x - 7

Now we multiply this difference by 4:
4(6x - 7)