2/7 + 1/3 + Unlnown side= 1/1
1/1 simply represents the whole circle. We need to find common multiples of all the numbers and change the denominator while also doing the same mathematical steps to the numerator: for example if you divide the denominator by 2 you also need to divide the numerator by 2. We end up getting 6/21 + 7/21 + Unknown/21 = 21/21
We get 13/21 + unknown = 21/21. We can solve this and get Unknown number = 8/21
