So... hmmm dunno either planet, I assume they're on the solar system and therefore orbiting the same planet, the sun, or whoever.
but, in short, what I can make out is, when do 6 and 9 have the same LCM, least common multiple, or a factor that's common to both 6 and 9, and therefore meet at such.
well, let's do a prime factoring of 6, is 2 and 3 only, ok,
now a prime factoring of 9, is 3 and 3 only, ok.
now, we have 3 as common to both, so, to get an LCD or LCM, we'll use repeated factors but once only for each instance for each.
therefore
so, after 18 years.
but, in short, what I can make out is, when do 6 and 9 have the same LCM, least common multiple, or a factor that's common to both 6 and 9, and therefore meet at such.
well, let's do a prime factoring of 6, is 2 and 3 only, ok,
now a prime factoring of 9, is 3 and 3 only, ok.
now, we have 3 as common to both, so, to get an LCD or LCM, we'll use repeated factors but once only for each instance for each.
therefore
so, after 18 years.