So.. .hmm let us see there are well.. in a MYR a million years, you said it :)
so 1,000,000y/myr or you can flip it around as myr/1,000,000y
for the sake of ratio, either one is valid
you can say, 1,000,000 years to a MYR or
one MYR to 1,000,000 years
same thing
now... let's see km... well, there are 1,000meters to 1km
and there are 100cm to 1meter
so.. let us use those ratios, flipped around as needed
cancel the units below and above as needed m/m cancels out, km/km cancels out, and so on