To simplify the expression \((3y^2)(6y^3)\), we can follow these steps:
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Multiply the coefficients (numerical parts): \[ 3 \times 6 = 18 \]
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Multiply the variables with the same base by adding their exponents: \[ y^2 \times y^3 = y^{2 + 3} = y^5 \]
Putting it all together, we have: \[ (3y^2)(6y^3) = 18y^5 \]
So the simplified expression is: \[ \boxed{18y^5} \]