I'm working on the same thing right now and I'm using every resource in my hands right now to find a solution. I will edit this post when I have more. If you need any more help I have completed the previous activity and most of this one.
EDIT:
Ok so I checked with somebody who is pretty good with math and they said that 5x=5x would be right. Not too sure but that's what I'm putting.
I put EVERYTHING for this assignment in comments down below. Geez all of this for 5 freakin points.
Answer:
x=3
please mark as brainliest
so 21 - 13, the money will be invested for 8 years.

I feel like there is something missing from this question, for example how large is the tree house? If these are just plain measurements, then I would opt for B.