Answer
2.21, 4.07
Work Step by Step
$\begin{align}
5\cos\theta+3&=0\\
5\cos\theta&=-3\\
\cos\theta&=-\frac{3}{5}
\end{align}$
Since $\cos \theta$ is negative in the second and third quadrants, the only values of $\theta$ in the interval $[0, 2\pi)$ that satisfy the equation are $\cos^{-1}(-\frac{3}{5})\approx 2.21$ and $2\pi-\cos^{-1}(-\frac{3}{5})\approx 4.07$.
