You need at least two numbers to find an LCM.
The common denominator is 28.
A common denominator, though not the least, is 168.
A common denominator, though not the least one, is 14*49 = 686.
A common denominator, though not the least one, is 14*12 = 168.
The least common denominator is 70.
There is none because the Least Common Denominator (or LCD) is the Least Common Multiple (LCM) of the denominators of a pair of fractions.
You need at least one more fraction in order to have a common denominator.
7.
Their product.
98
168
126