To simplify the expression \( y^{-9} \cdot y^{-3} \cdot y^{-5} \), you can use the property of exponents that states when you multiply like bases, you add the exponents:
\[ y^a \cdot y^b = y^{a+b} \]
In this case, you add the exponents \(-9\), \(-3\), and \(-5\):
\[ -9 + (-3) + (-5) = -9 - 3 - 5 = -17 \]
So, the expression simplifies to:
\[ y^{-17} \]
If you want to express this with a positive exponent, you can write it as:
\[ \frac{1}{y^{17}} \]
Therefore, the final simplified form of the expression is:
\[ y^{-9} \cdot y^{-3} \cdot y^{-5} = y^{-17} = \frac{1}{y^{17}} \]