We are trying to find the LCM, or "Least Common Multiple", of 3 and 8
What this means is that we need to find the least number that both numbers can get to.
Lets write out the multiples for each number:
3: 8:
3 16
6 24 < --- 8 has a multiple of 24
9 32
12 40
15
18
21
24 <---- 3 has a multiple of 24
27
30
You would notice that both numbers can make 24.
This means that the LCM of 3 and 4 is 24