Answer:
21y
Explanation:
Well, first take a look at what exactly an LCD is. You want the lowest common multiple of the denominators aka the bottoms of the fractions.
The two denominators you have are 3y and 7y. What multiplication or division can you do to make these two equal? Go through your multiples of 3 and 7 (think times tables) and see where they finally have one in common.
3, 6, 9, 12, 15, 18, 21, 24...
7, 14, 21, 28, 35...
So, between these two lists, we can see that the lowest multiple these two numbers have in common is 21. You could go higher and higher, but your question wants the lowest, so only 21 will do that for you. To make 3y and 7y equal, you need to multiply your first term by 7/7 and your second term by 3/3. If you do that, you'll see your least common denominator is 21y.