To simplify the expression \((y^{-4} \cdot y^{7})^{7}\), we can start by using the properties of exponents.
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Combine the exponents inside the parentheses: \[ y^{-4} \cdot y^{7} = y^{-4 + 7} = y^{3} \]
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Now simplify the expression raised to the power of 7: \[ (y^{3})^{7} = y^{3 \cdot 7} = y^{21} \]
Therefore, the simplified form of the original expression \((y^{-4} \cdot y^{7})^{7}\) is:
\[ \boxed{y^{21}} \]