In both lists of factors, 4 is in common. Therefore, the value of K is 4 because it can divide into both 12 and 20 without a remainder. 2 is also another possible value of K.
So first you would have to make the two have common denominators. What we can do for that is say that they both go into 12. 1/6 x 2 = 2/12 and 3/4 x 3 = 9/12. Then we can add the two which is 2/12 + 9/12 = 11/12.