#### Answer

$(-9)^{4}$

#### Work Step by Step

In algebra, exponents can be used to write products of repeated factors. Exponents take the form $a^{b}$, where $a$ is the repeated factor and $b$ is the amount of times that the factor is being multiplied by itself.
In this case, -9 is being multiplied by itself 4 times.
Therefore, $(-9)(-9)(-9)(-9)=(-9)^{4}$.