Q_Htgt=5; % the target solution value
fnQ=@(QG) getGUE(T_hwi,T_set,T_ext,QG)-Q_Htgt; % define solver function
Q_guess = YourStartGuessValue; % need a starting value in neighborhood
Q=fzer0(@fnQ,Q_guess); % see if can find a zero
[~,GUE_tgt,COP_tgt]=getGUE(T_hwi,T_set,T_ext,Q); % solve for other values given the zero
NB: The values of the other arguments to getGUE and the target Q_H values are embedded in the anonymous function definition for fnQ; if you need to solve for another set of parameters or target value, those values must be redefined and then the definition of the anonymous function re-executed to make it reflect the changes.
See doc fzero for more details, examples with alternate ways to handle extra parameters.
