how to crop image using gui

i have a problem, i have to crop object on photos using imcrop but i want crop with fixed ratio using edit text... in this photo, the variabel are X max and Y max. if we are not using gui, the code will be : I2 = imcrop(I,[75 68 a b]); a is X max and b is Y max

because i'm using gui my code will be like this :

I = handles.Citra.RGB;

[I2 rect] = imcrop(I);
axes(handles.axes2);
imshow(I2);
guidata(hObject,handles);

how do I make the code using the edit text as a variable cropping the objects. please help me to solve this. i will waiting for anyone to answer my question