function u = CalcBlockCase6(x,v,d,p)
% model for level and concentration 
q2   = d(1);                           % [kg/s], inlet flow 2
c1    = d(2)*1;                        % [kg/kg] inlet flow 1 concentration 
c2    = d(3)*1;                        % [kg/kg] inlet flow 2 concentration 
c    = x;                              % [kg/kg], concentration
%backcalculate u from v
u = (v - q2*(c2-c))/(c1-c);
end