(1 + 2 cos x)(1 - 3 cos x) = 0

1 + 2 cos x = 0 or 1 - 3 cos x = 0

cos x = -1/2 or cos x = 1/3

x = ±2π/3 + 2πn, n ∈ Z or x = ±arccos(1/3) + 2πn, n ∈ Z