Answer: 17cm.
Step-by-step explanation:
The equation you're using is:
Δd = df - di
Which means the change in position is equal to the final position minus the starting position. In this case that works out to 20cm - 3cm = 17cm. We're only interested in how much the snail moved, not how long it took to move, so even though they give a time it actually doesn't matter for this question.