-6arcsin3x=pi

-6arcsin(3x) = π

Divide both sides by -6:

arcsin(3x)  =  - π / 6

Take the sin of both sides:

  3x  =  sin( - π / 6)

But sin( - π / 6) = -sin( π / 6)   and sin( π / 6) = 1/2, so:

  3x  =  -1/2

   x  =  -1/6