What is the common denominator for 1/3 and 1/4

12 because if u multiply 3*4 it is 12 and 4*3 is 12 so they both have 12 in common.

To find common denominators 
you may take each denominator and provide the acceptable factors.

factors of 3: 1 , 3 , 6 , 9 , 12ETC
factors of 4: 1 , 2 , 4 , 8 , 12...ETC

Note that they both have a common number of 12 

So you will multiply the 1st fraction by 4 and the 2nd by 3

4(1/3) = 4/12
3(1/4) = 3/12

no need for the last part unless if you are proceeding with different properties.