Best way to work out planet torrid take 6 years to complete one orbit , planet monde blanc takes 9 years in the year 2035 both planets are lined up perfectly. how many years will it be before the two worlds are line up again ?

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.

Welp life just is hard