Calculating the Year Length of Planet Y Around Alpha

[jnimagine]

Planet X orbits the star Alpha with a 'year' that is 200 Earth days long. Planet Y circles Alpha at nine times the distance of planet X. How long is a year on planet Y?

I know that I have to use the kepler's law : T^2 = (4pi^2/GM)r^3
but I'm not too sure how to do solve this...
Please help!
Thanks.

 

[LowlyPion]

Planet X orbits the star Alpha with a 'year' that is 200 Earth days long. Planet Y circles Alpha at nine times the distance of planet X. How long is a year on planet Y?

I know that I have to use the kepler's law : T^2 = (4pi^2/GM)r^3
but I'm not too sure how to do solve this...
Please help!
Thanks.

The third law : "The squares of the orbital periods of planets are directly proportional to the cubes of the axes of the orbits."

So T1²/r1³ = T2²/r2³