Speed is the distance traveled in unit time. Or speed = distance/time which gives distance = speed*time.
The speed of the snail is 8 cm/min.
In 3 minutes, the snail can travel 8*3 = 24 cm
3 hours is equal to 3*60 = 180 minutes. In 180 minutes the snail can travel 180*8 = 1440 cm
The snail can travel 24 cm in 3 minutes and 1440 cm in 3 hours.
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.