To simplify the expression \( y^{-5} \cdot y^{-8} \cdot y^{-7} \), we can use the property of exponents that states when multiplying two powers with the same base, we add their exponents:
\[ y^{-5} \cdot y^{-8} \cdot y^{-7} = y^{-5 + (-8) + (-7)} \]
Now, let's add the exponents:
\[ -5 + (-8) + (-7) = -5 - 8 - 7 = -20 \]
So we have:
\[ y^{-5} \cdot y^{-8} \cdot y^{-7} = y^{-20} \]
To write this in exponential form with positive exponents only, we convert \( y^{-20} \) to:
\[ \frac{1}{y^{20}} \]
Thus, the fully simplified expression is:
\[ \frac{1}{y^{20}} \]