LCM( 3, 21 ) = 21
The formula for finding LCM is
$$ \text{LCM} (a, b) = \frac{ a \cdot b }{ \text{GCD}(a,b)} $$In this example we have: a = 3, b = 21 and GCD(3, 21) = 3.
$$ \text{LCM} (3, 21) = \frac{ 3 \cdot 21 }{ \text{GCD}(3,21)} = \frac{ 63 }{ 3 } = 21 $$