As Adam said you need to get the input variables from the GUI handles. As soon as the first pushbutton callback is complete, a, b, c, and d don't exist anymore.
You want something like this in the callback for pushbutton2
a = handles.input1;
b = handles.input2;
You should also note that as you have written it, aa, bb, and cc are strings, so you calculations will all be wrong (try 2*'1' in the command line).You need to convert them to numbers...
aa = str2double(get(handles.edit1,'String'));
bb = str2double(get(handles.edit2,'String'));
cc = str2double(get(handles.edit3,'String'));
Also, if you are writing a serious GUI you should take time to make sure the the user inputs are correct. If the user types in a letter, str2double() will return NaN. It is worthwhile adding default values that replace any silly user input.