Answer
$f(x)=kx+C$, a series of lines.
Work Step by Step
Step 1. Assuming $g(x)=kx$ where $k$ is a constant, we have $g'(x)=k$.
Step 2. Based on Corollary 2, if any other function $f(x)$ satisfies $f'(x)=g'(x)=k$, we have $f(x)=g(x)+C=kx+C$, where $C$ is a constant.
Step 3. Thus we conclude that for any function whose derivative is a constant, it should have an equation in the form of $f(x)=kx+C$, which represents a series of lines.
