#### Answer

$ F(x)=-3^{x}$

#### Work Step by Step

We first find the graph of $ f(x)=3^{x}$ among the graphs given from 19 to 24.
The graph of $ f(x)=3^{x}$ is
- always above the x-axis, (graphs 20, 22, 24)
- from left to right, always rises and passes through (0,1) and (1,3)
The graph of $ f(x)=3^{x}$ is graph 22.
This graph (graph $21$) is obtained from the graph of $ f(x)=3^{x}$ by
reflecting it about the x-axis: $-f(x)=-3^{x}$
$ F(x)=-3^{x}$