What Is the Orbital Period of an Asteroid with Twice Earth's Mean Radius?

[DRM126]

Homework Statement

An asteroid revolves around the sun with a mean orbital radius twice that of Earth's. Predict the period of the asteroid in Earth years

Homework Equations

I guess t^2/r^3 where t = period and r = mean orbital radius

The Attempt at a Solution

I missed the lecture in class, and I'm ultimately confused on the subject of Orbital Dynamics and Kepler's Law, any help/explanation would be great, or a link to a useful website, honestly I'm clueless. I think the fact that the r for the asteroid is twice as long as Earth's creates a ratio?

 

[fluidistic]

Check out http://en.wikipedia.org/wiki/Kepler's_laws. I cannot enter the website right now but if I remember well it had an example of the third law I think.