The least common denominator is the Least Common Multiple (LCM) of the divisors 3 and 4. Since these numbers have no common factors, their LCM is
... 3·4 = 12
The least common denominator of your fractions is 12.
... 1/3 = (1/3)·(4/4) = 4/12
... 3/4 = (3/4)·(3/3) = 9/12
