To simplify the expression \( 6y^4(2y^2 - y) \), we can distribute \( 6y^4 \) to both terms inside the parentheses.
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Distributing \( 6y^4 \) to \( 2y^2 \): \[ 6y^4 \cdot 2y^2 = 12y^{4+2} = 12y^6 \]
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Distributing \( 6y^4 \) to \( -y \): \[ 6y^4 \cdot (-y) = -6y^{4+1} = -6y^5 \]
Now, we put both results together: \[ 12y^6 - 6y^5 \]
Thus, the simplified form of the expression \( 6y^4(2y^2 - y) \) is: \[ \boxed{12y^6 - 6y^5} \]