% x 

/acos {
dup 			% x x
dup mul 		% x x*x
1 sub neg 		% x 1 - x*x
sqrt			% c s
exch atan
} def

% a b c: x^{3} + ax^{2} + bx + c = 0

/solve {
10 dict
begin
/c exch def
/b exch def
/a exch def
/B b a a mul 3 div sub def			% B = b - a^{2}/3
/C a a mul a mul 					% a^{3}
2 mul 27 div 						% 2a^{3}/27
a b mul 3 div sub 					% 2a^{3}/27 - ab/3
c add def							% C 
/alpha 4 B mul neg 3 div sqrt def
/cos3 4 C mul neg alpha div alpha div alpha div def
/theta cos3 acos 3 div def
theta cos
alpha mul 
a 3 div sub 
theta 120 add cos
alpha mul 
a 3 div sub 
theta 240 add cos
alpha mul 
a 3 div sub 
end
} def

-6 11 -6 solve stack