Answer:
21
Step-by-step explanation:
The denominators of 3 and 7 have no factors in common, so their least common multiple is their product: 3·7 = 21.
The LCD is 21.
_____
Additional comment
When the equation is multiplied by 21, it becomes ...
126y -6 = 7y . . . after multiplying by 21
119y = 6 . . . . . . . add 6-7y
y = 6/119
