9 is the sum of 7 and twice a number

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9 is the sum of 7 and twice a number

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

Twice a number means we multiply x by 2:
2x

The sum of 7 and twice a number
7 + 2x

The word is mean equal to, so we set 7 + 2x equal to 9:
7 + 2x = 9