Answer:
and 
Step-by-step explanation:
when you solve for r in the given equation, you need to apply the square root property, which gives positive and negative answers (both should therefore be considered):

then you need to include these two possible solutions:
and 
The answer seems to be D!!
Answer: B
Step-by-step explanation:
Since
, it follows that 
It will take 20 years for the shrub to reach a height of 7 feet.