Yep. Good job. You are now a scientist. The temperature can be determined by considering how much FeO there was in the solid material, how much FeO there was in the liquid material (which is now solid rock at the surface) and at what temperature that particular combination occurs. In real research, the problem is a little more complicated because the composition of the material inside the planet, the pressure, the amount of oxygen and many other things must be figured out at the same time as the temperature, because each of these things affects each of the others.