They are not fractions, so they do not have denominators.
The Least Common Denominator of 3/4 and 7/8 is 8.
The common denominator is 28.
The least common denominator (LCD) is 630.
For 5/6 and 7/8 the LCD is 24.
42
84
126
It is: 42
84
1008
42
420
It is 630
126
84
84/14 = 6 84/7 = 12 84/3 = 28 Hence 84 is the least common denominator