Final answer:
To simplify the given expression, gather like terms and combine their coefficients and constants. The simplified expression is x + 17.
Step-by-step explanation:
To simplify the expression '21 - 8x - 4 + 9x,' you need to gather like terms. Like terms are terms that have the same variable raised to the same power. In this expression, the like terms are -8x and 9x. To simplify, combine the coefficients of these terms and keep the variable the same.
- Combine the coefficients: -8x + 9x = 1x = x
- Combine the constants: 21 - 4 = 17
Therefore, the simplified expression is x + 17.
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