conplaneteering

I'm trying to calculate the orbital period of an Earth-sized planet at
15 AUs out from a star that's about 7.5 times the mass of the Sun.  I'm
guestimating around 50 years, but Kepler's laws don't seem to be much
help when looking for absolutes; you need a known quantity within the
same solar system to compare against.  Can anyone point me at the
correct formulae?

Thanks.

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