The given expression is:

$\left(3r\right)^2$

To simplify, apply the exponent to both terms inside the parentheses:

$(3r)^2 = 3^2 \cdot r^2$

$= 9r^2$

Thus, the simplified form of $\left(3r\right)^2$ is $9r^2$.

Would you like further details or have any questions?

Related Questions:

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5. How do you simplify $\left(2x^3y\right)^4$?

Tip: When an exponent is applied to a product inside parentheses, distribute the exponent to each term separately.