To simplify the expression \((Y^3)^7 \cdot (y^7)^5\), we can use the power of a power property of exponents, which states that \((a^m)^n = a^{m \cdot n}\).
Applying that property, we have:
- \((Y^3)^7 = Y^{3 \cdot 7} = Y^{21}\)
- \((y^7)^5 = y^{7 \cdot 5} = y^{35}\)
Now we can combine the two results:
\[ Y^{21} \cdot y^{35} \]
Since \(Y\) and \(y\) are different variables, we cannot combine them further. Thus, the final simplified expression is:
\[ Y^{21}y^{35} \]