Choice Choice C is the correct answer.

1. Start with the equation: $3x - 9 = 21$
2. Add 9 to both sides: $3x = 21 + 9$
3. Simplify: $3x = 30$
4. Divide both sides by 3: $x = \frac{30}{3}$
5. Simplify: $x = 10$
6. Verify: $3(10) - 9 = 30 - 9 = 21$ ✓

Choice Choice C is incorrect because this results from subtracting 9 from 21: $12$, then $12/3 = 4$. We must add to undo subtraction.

Choice Choice B is incorrect because this is the value of $3x$. Don't stop early!

Choice Choice D is incorrect because this results from multiplying 30 by 3: $90$.