3 times a number increased by 1 is between -8 and 13

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3 times a number increased by 1 is between -8 and 13.

Let's take this algebraic expression in 4 parts:

Part 1 - The phrase a number means an arbitrary variable, let's call it x:
x

Part 2 - 3 times this number means we multiply x by 3:
3x

Part 3 - Increased by 1 means we add 1 to 3x:
3x + 1

The phrase between means we have an inequality:
-8 <= 3x + 1 <=13