Maxima 5.5 ...
(C1) f(x):=x^3-10*x^2+28.5*x-21;
(D1) ...
(C2) rhs(solve(f(x),x)[1]),rectform,numer;
(D2) 12.01301985645547*%I+10.38424295325536
which is wrong.
Maxima 5.5 ...
(C1) f(x):=x^3-10*x^2+28.5*x-21;
(D1) ...
(C2) numer:true;
(D2) ...
(C3) solve(f(x),x)[1],fullratsimp;
(D3) ...
(C4) expand(%);
(D4) x=12.01301985645547*%I+10.38424295325536
and the result is the same one.
To R.Fateman: Yes, I try your code and it is all right.
To M.Rubey: I was surprised to see that your result is a different one.
Thank you! May be this is enough.
M.At.Stanev