Answer
$2.42,3.86$
Work Step by Step
Use a calculator, $4cos\theta+3=0 \Longrightarrow cos\theta=-\frac{3}{4} \Longrightarrow \theta=cos^{-1}(-\frac{3}{4})=\pi-cos^{-1}(\frac{3}{4})\approx2.42$ and $\pi+cos^{-1}(\frac{3}{4})\approx3.86$ in $[0,2\pi)$.
