LCM( 39, 42 ) = 546
The formula for finding LCM is
$$ \text{LCM} (a, b) = \frac{ a \cdot b }{ \text{GCD}(a,b)} $$In this example we have: a = 39, b = 42 and GCD(39, 42) = 3.
$$ \text{LCM} (39, 42) = \frac{ 39 \cdot 42 }{ \text{GCD}(39,42)} = \frac{ 1638 }{ 3 } = 546 $$