Answer
$100$.
Work Step by Step
The given expression for the number of bacteria is
$=900\cdot 3^x$
Substitute $x=-2$.
$=900\cdot 3^{(-2)}$
Use power rule.
$=\frac{900}{ 3^{2}}$
Simplify.
$=\frac{900}{ 9}$
$=100$.
Hence, the number of bacteria for $x=-2$ is $100$.
