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Explanation:
The curly braces tell us we're dealing with a set of values.
The notation [tex]x \in \mathbb{R}[/tex] means "x is in the set of real numbers". In other words, it says "x is a real number".
Writing [tex]17 \le x \le 21[/tex] tells the reader that x is between 17 and 21, including both endpoints.
Put everything together and [tex]\{x|x\in\mathbb{R} \text{ and } 17 \le x \le 21\}[/tex] means "we have a set of real numbers x that are between 17 and 21, inclusive".
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