What’s the least common multiple for each group of numbers?
a. 6 and 15
b. 4 and 11
c. 6, 9, and 12
d. 8, 10, and 20

You have to use the prime factorization
A, 6= 2*3   15=3*5
You can see that three is common in both so you do not need both
LCM=2*3*5=30

B, 4=2*2    11 is a prime so 
LCM=4*11=44

Hope this will help you solve C and D too. 

The expression -8/-4 has the same value as -18/-9.