Answer
True, $f(x)=f(-1)=3$ for all $x$.
Work Step by Step
By Corollary 1, we know that if $f'(x)=0$ on an open interval, then $f(x)=C$, where C is a constant. Since we know that $f'(x)=0$, we must have $f(x)=C$ and $C=3$ because $f(-1)=3$. Thus $f(x)=3$ for all $x$. True.
