(-a)5 • (-a)4

To simplify the expression \((-a)^5 \cdot (-a)^4\), you can use the property of exponents that states:

\[ x^m \cdot x^n = x^{m+n} \]

Here, \(x = -a\), \(m = 5\), and \(n = 4\). Applying the property:

\[ (-a)^5 \cdot (-a)^4 = (-a)^{5+4} = (-a)^9 \]

So the simplified form of the expression is:

\[ (-a)^9 \]

If needed, you can also express it as:

\[ -(a^9) \]

depending on the context.