Work with your units:
1 watt-hour = 1 (joule/second) · (hour) = 1 (joule-hour / second)
(1 joule-hour/sec) · (3600 sec/hour) = 3600 joules
So 1 watt-hour = 3,600 joules
The Kepler's laws predict the planetary motion, so there are three laws for this, namely:
1. The orbit of a planet is an ellipse with the Sun (the sun is a star!) at one of the two focus.
2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
3. The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.
So, let's use second law. The Sun sweeps out equal areas during equal intervals of time means that if A = B, the time the planet takes to travel A1A2 is equal to the time the planet takes to travel B1B2, but given that A = 2B, then takes twice the time to travel A1A2 compared to B1B2.