the 2 and the 1 are complete wholes so they can be added easily 2 + 1 = 3
then you have 1/2 and 1/6 you need to find a common denominator
1/2 is equal to 2/4,4/8,3/6.... but we can use 3/6 for 1/2
3/6 + 1/6 = 4/6 to simplify it the numerator and the denominator have to share a common factor, in this case, 2. 4/6 can be condensed into 2/3