2sin(3x) + 3cos(3x) = 0
2sin(3x) = -3cos(3x)
sin(3x)/cos(3x) = -3/2
tan(3x) = -3/2
3x = tan-1(-3/2)
x = (1/3) tan-1(-3/2)
Use your calculator to find tan-1(-3/2). It will give you a negative angle value, which is not in the range 0 to 2π ( 0 to 360°). The tangent function repeats every 180°, so just keep adding 180 to the value from the calculator to find the values for x in the interval 0 to 360°.

Doug C.
And if you are able to use Desmos, here is how you can achieve that result there: desmos.com/calculator/dmc7avu7rb07/09/21