Finding an asteroid's semi-major axis from its orbital period

[ghostbuster25]

my question is... the semimajor axis of the orbit of Jupiter is 7.8*108.
If an asteroid has an orbital period half of that of Jupiter what is the semi major axis?

All i did was times 7.8*108 by 1/2 and got 3.9*108.

Seems to simple!??

any comments much appreciated

 

[D H]

What does Kepler's third law say?

 

[ghostbuster25]

that the square of the orbital period is propotional to the square of the semi major axis.

im unsure of how to apply this though.

I believe T²/a³=K to be relivent but as i only know that the semi major of the sun and Jupiter is 7.8*10⁸ I am confused what to do next