sin 3x=-1

sin 3x = -1

I know sin 270° = -1
so 3x = 270
x = 90° or in radians : x = π/2

the period of sin 3x is 360/3 = 120°
so more solutions are obtained by adding or subtracting multiples of 120° to any answer we have
so x = 90°, 210°, 330° , 0 ≤ x ≤ 360°

I leave it up to you to express in radians if you have to