How do I compute for the transfer function of a bandpass 6th order butterworth filter?

I currently have this code

fs=3000; % Sampling rate filterOrder=6; % Order of filter

if true
  % code
end
passBand=[650, 800];
[b, a]=butter(filterOrder, passBand/(fs/2));
[h, w]=freqz(b, a);
plot(w/pi*fs/2, abs(h), '.-');
xlabel('Freq (Hz)'); title('Freq. response of the band-pass filter'); grid on