Answer
$\frac{2\pi}{9},\frac{4\pi}{9},\frac{8\pi}{9} $
Work Step by Step
$f(x)=0 \longrightarrow 2cos(3x)+1=0 \longrightarrow cos(3x)=-\frac{1}{2} \longrightarrow 3x=\frac{2\pi}{3},\frac{4\pi}{3}, \frac{8\pi}{3} \longrightarrow x=\frac{2\pi}{9},\frac{4\pi}{9},\frac{8\pi}{9} $
