There's a self-test at the end of each chapter of my EE textbook, and I couldn't figure out--because I didn't understand--how the correct response was arrived at in the following question. "A certain circuit produces 15 V across its open output terminals, and when a 10K load is connected across its output terminals, it produces 12 V. The Thevenin equivalent for this circuit is...(and the answer is 15 V, 2.5K Ohms). I didn't know where to begin to figure the answer out, and neither does my tutor. My instructor goes way too fast for people taking their first electronics course. Anyway, thanks for any help.