For one equation ==> Warning: Explicit solution could not be found

Hi,

I've written 4 kind of codes to solve lam:

%-----------1----------------
_syms lam K4 Vw;
S=solve((119/lam - 9)*exp(-21/lam)-K4/Vw==0,lam)_
%-----------1----------------

%------------2---------------
_syms c1 c2 c3 lam K4 Vw;
S=solve((c1/lam - c2)*exp(-c3/lam)-K4/Vw==0,lam)_
%------------2---------------

%------------3---------------
_syms c1 c2 c3 lam K4 Vw;
S=solve((c1/lam - c2)*exp(-c3/lam)==K4/Vw,lam)_ 
%------------3---------------

%------------4---------------
_syms c1 c2 c3 lam K4 Vw;
S=solve((c1/lam - c2)*exp(-c3/lam)==K4/Vw)_ 
%------------4---------------

The answer is always the same: Warning: Explicit solution could not be found

Help me please!