Choice Choice D is the correct answer. To isolate the variable, we divide by the coefficient.

1. Start with the equation: $9x = 81$
2. Divide both sides by 9: $x = \frac{81}{9}$
3. Simplify: $x = 9$
4. Verify: $9(9) = 81$ ✓

This is a perfect square relationship: $9^2 = 81$, so when we divide 81 by 9, we get 9 back.

💡 Pattern Recognition: Recognizing that $81 = 9 \times 9$ helps you solve this mentally.

Choice Choice B is incorrect because this results from subtracting: $81 - 9 = 72$. Subtraction is not the inverse of multiplication.

Choice Choice C is incorrect because this comes from adding: $81 + 9 = 90$. Addition doesn't undo multiplication.

Choice Choice A is incorrect because this results from multiplying: $81 \times 9 = 729$. This compounds the multiplication instead of reversing it.