The north american and european continents are moving apart at a rate of about 3 cm/y. at this rate how long will it take them to drift 500 km farther apart than they are at present?

In this case, the continents are moving apart at a rate (velocity = 3 cm/y) and the distance for continent to move (distance = 500km). You are asked how long it will take (time). Then to answer this question you need to convert the 500km into the same unit as the velocity which was cm. The formula would be:

Distance = velocity x time

500km x 10^5 cm/km = 3cm/year x time

time = 5x 10^7 cm / (3cm/year)

time = 1.7 x 10^7 year or 17 million year