the difference between triple a number and double a number

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the difference between triple a number and double a number

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

Triple a number means we multiply x by 3:
3x

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

The difference means we subtract 2x from 3x:
3x - 2x

Simplifying like terms, we have:
(3 - 2)x = x