28 is the LCD. You can always get a common denominator by dividing the numbers 7 x 4 = 28.
To find the LCD I look at the multiples of the larger number(since there will be fewer of them) As I do this I check to see if the other number is a factor of it
7(4 isn't a factor), 14(4 isn't a factor), 21(4 isn't a factor), 28(4 IS a factor)!!!