The sum of 6 times a number and -8, added to 3 times a number

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The sum of 6 times a number and -8, added to 3 times a number

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

6 times a number:
6x

And means we add, so we have
6x - 8

Added to 3 times a number
6x - 8 + 3x

Combine like terms:
9x - 8