y^-9*y^-3*y^-5=

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:

\[
y^{-9} \cdot y^{-3} \cdot y^{-5} = y^{-17} = \frac{1}{y^{17}}
\]