please help i'm really stuck

Answer:
The scale factor is 1 : 3, and the missing length is 8
Step-by-step explanation:
The scale factor: 7/21 = 1/3, 6/18 = 1/3
The missing length: 24/3 = 8 or 8/24= 1/3
If it is right can you please give me a brainliest. Pls. It is fine if you didn't bu here's the answer