Answer
$2.42,3.86$
Work Step by Step
$4cos\theta+3=0 \longrightarrow cos\theta=-\frac{3}{4} \longrightarrow \theta=cos^{-1}(-\frac{3}{4})=\pi\pm cos^{-1}(\frac{3}{4})\approx\pi\pm0.723$. Thus $\theta\approx2.42,3.86$
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