Which of the following has the same value as $$81x^2$$?

Correct.
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Notice that $$81 = 9^2$$. Thus, $$81x^2 = 9^2x^2$$.
Since both powers have the same exponent, combine both bases under the same power.
>$$9^2x^2 = (9x)^2$$
Hence, this is the correct answer.

Incorrect.
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Incorrect.
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Incorrect.
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Incorrect.
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$$3x^2$$

$$(3x)^2$$

$$(3x)^4$$

$$9x^2$$

$$(9x)^2$$