Factors of 12 : 1, 2, 3, 4, 6, 12
Factors of 20: 1, 2, 4, 5, 10, 20
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.
Inserting all the given information into the identity, we have

It is just a dark screen. Upload it again?
To subtract fractions, find the LCD and then combine.
Answer is: -3