Set 5: Linear Equations

Choice A is correct. 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 C is incorrect because this results from subtracting 9 from 21: $12$, then $12 /3 = 4$. We must add to undo subtraction. Choice B is incorrect because this is the value of $3x$. Don't stop early! Choice D is incorrect because this results from multiplying 30 by 3: $90$.