Hi Harps,
This is such a commonly asked question that you'd be best served by using the forum's search facility first. There must be thousands of threads that cover this area.
You see, you've asked one of those 'simple' questions that actually requires quite a complicated answer!
If you can't find out what you need by searching, then come back here and provide as much information as possible about your combo, such as:
1) What make/model it is
2) Its rated power output in RMS Watts
3) The impedance of its built-in speaker (if you know)
4) Whether it is a solid state or tube amp
Armed with that information, whoever replies will be able to give you a proper answer that will make sense to you.