Tell me the answer! i'm crying in confusion! : (

First, change the fractions to a common denominator, which is 21. 

so 3/7 * 3/3  = 9/21

1/3 * 7/7 = 7/21

Add the two together

9/21 + 7/21 = 16/21

Then take 21/21 - 16/21 = 5/21

There is 5/21 of the pizza left.

Find the common denominator:

3   6   9
--  --   --  
7  14  21

1  2  3  4  5  6      7     8
--  -- -- -- -- -- --   ---    ---
3  6  9  12  15    18    21

Ok, so we know the common demonators are 9/21 and 8/21

Now we subtract  9/21 and 8/21 which is 1/21 

So.. There is 1/21 pizza left!

Hope this helped :))

The answer should be c.