Answer:
18
Explanation:
1. Create the ratio 12:9
The first number in the ratio is where the length of the shadow will go. The second number is where the length of the building will go.
2. Subtract 21 by 12 to find out how much we have to add to 9. (21-12=9)
3. Add 9 to 9. (18)
We now have 21:18.