The LCM or least common multiple is the smallest possible number or in this case expression that can be divided by both terms. We factor out each expressions first giving us,
21ab²c³ = 7.3.a.b.b.c.c.c
18a²b³c = 3.3.2.a.a.b.b.b.c
Then, we copy the factors for our LCM, making sure not to rewrite the repetitions.
LCM = 7.3.3.2.a.a.b.b.b.c.c.c
The answer should be
LCM = 126a²b³c³
