Start with x , subtract 6, then times by 3.

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Start with x , subtract 6, then times by 3.

We start with x:
x

Subtract 6:
x - 6

The phrase times by means we multiply (x - 6) by 3
3(x - 6) <-- This is our algebraic expression

If the problem asks you to multiply through, then you'd have:
3x - 18