Answer
$\{2.5136, 3.7696 \}$
Work Step by Step
Step 1. Given $cos(x)=-0.8092$, we have the reference angle $x_0=cos^{-1}(0.8092)\approx0.6280\ rad$
Step 2. We can find the first solutions as $x_1=\pi-x_0\approx2.5136$
Step 3. We can find the second solutions as $x_2=\pi+x_0\approx3.7696$
Step 4. We have the solution set as $\{2.5136, 3.7696 \}$
