Distance = velocity x time, so for the first scenario
8cm/min x 3 min = 24 cm traveled.
There are 60 minutes in one hour, so 3 hours x 60 minutes = 180 minutes.
8 cm/min x 180 minutes = 1440 cm traveled
This assumes the snail is crawling in a straight line, of course - if it changes direction then we have to figure out whether to answer the question with distance, which is simply an expression of how much ground the snail has covered (like the odometer reading on a car), or displacement, which tells us how far the snail is from its starting point, measured in a straight line (regardless of whether the snail's path was straight or not).
The first link below explains this idea in more detail. The second link will take you to a nifty calculator that will actually do these problems for you, as long as you input the data correctly.Always pay attention to the units when you are doing these.