Answer:
100%
Explanation:
The tree grew from 6 feet to 12 feet, which is a growth of 6 feet. 6 feet/6feet is 1, which in percent is 100%.
Now, you could say as the tree doubled in size, the change is 200%, however you must subtract the original height to gain the amount of change. So, 12-6 = 6, and 6/6 = 1, 1*100 = 100%.