3 times a number is 3 more a number

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3 times a number is 3 more a number

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

3 times a number:
3x

3 more than a number means we add 3 to x:
x + 3

The word is means and equation, so we set 3x equal to x + 3
3x = x + 3