`cos θ = -0.9135` (`0 ≤ θ < 2π`)
We first find the relevant acute angle by solving the positive case, `cos α = 0.9135` (which is what we were doing in the "Reference Angle" section in 6. Trigonometric Functions of Any Angle).
`α = 0.41899` radians
Since `cos θ` is negative, it means θ is in the second and third quadrants.
`θ = π - 0.41899 = 2.723`, or
`θ = π + 0.41899 = 3.561`