3 csc x cot^2 x = csc x
3 csc x cot^2 x - csc x = 0
csc(x)[3cot2(x) - 1] = 0
This only can happen if the two factors equal 0 so:
csc(x) = 0 (no solution because csc(x) is always > 1)
and
3cot2(x) - 1 = 0
3cot2(x) = 1
cot2(x) = 1/3
cot(x) = ± 1/√3
Looking at the unit circle, the only solution between π/2 and π is when cot(x) = -1/√3 at 2π/3
cot(2π/3) = adj/opp = (-1/2)/(√3/2) = -1/√3