Multiply or divide as indicated. y -9 • y -8 • y 10

y⁻⁷

Explanation:
Before we begin, remember the following rule:
1- mᵃ * mᵇ = mᵃ⁺ᵇ
2- mᵃ / mᵇ = mᵃ⁻ᵇ

Now, for the given we have:
y⁻⁹ * y⁻⁸ * y¹⁰
We can note that we are multiplying variables of same base with different powers. Therefore we will simply apply the first rule mentioned above and add the powers as follows:
y⁻⁹ * y⁻⁸ * y¹⁰ = y⁻⁹⁺⁽⁻⁸⁾⁺¹⁰ 
                      = y⁻⁷

Hope this helps :)