nursuhailah - in the push button callback, you would get the value of the pop-up menu and the resistance value text, and do the appropriate calculation. For example,
function pushbutton1_Callback(hObject, eventdata, handles)
% get the maximum power
maxPower = 0;
maxPowerIdx = get(handles.popupmenu1,'Value');
if maxPowerIdx == 1
maxPower = 2;
elseif maxPowerIdx == 2
maxPower = 4;
end
% get the resistance
valueOfResistance = str2double(char(get(handles.edit1,'String')));
% do the calculation
voltage = maxPower * valueOfResistance;
% set the resistance
set(handles.text1,'String',num2str(voltage));
In the above, I'm assuming that your pushbutton is named pushbutton1, the popup menu is named popupmenu1, the resistance edit text field is named edit1, and the static text field where you are writing your answer is named text1. The above is untested but should give you an idea of how to proceed.
