Use Kepler's third law to determine how many days it takes a spacecraft to travel in an elliptical orbit from a point 6 647 km from the Earth's center to the Moon, 385 000 km from the Earth's center.

I know the formula for keplers third law and that the radius or a in this case is supposed to be the semi-major axis, I just have failed to figure out how to get the semi-major axis.