LCM( 12, 21 ) = 84
The formula for finding LCM is
$$ \text{LCM} (a, b) = \frac{ a \cdot b }{ \text{GCD}(a,b)} $$In this example we have: a = 12, b = 21 and GCD(12, 21) = 3.
$$ \text{LCM} (12, 21) = \frac{ 12 \cdot 21 }{ \text{GCD}(12,21)} = \frac{ 252 }{ 3 } = 84 $$