Wash the rutabagas and carrots. Peel the rutabagas, but not the carrots. Cut both of the veggies into 1 inch cubes. Cook the rutabagas in a large pot of salted water until very tender, about 45 minutes. Add the carrots toward the end, so that they cook for 5-10 minutes. Drain well. Return to the pot and mash together. Stir over medium heat and add the butter. Add salt & pepper if you like.