Sin3x = -√2/2

3x = (-1)ⁿarcsin(-√2/2) + πn, n ∈ Z

3x = (-1)ⁿ⁺¹π/4 + πn, n ∈ Z

x =  (-1)ⁿ⁺¹π/12 + πn/3, n ∈ Z

Ответ: x =  (-1)ⁿ⁺¹π/12 + πn/3, n ∈ Z.