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.
The equation must equal 84, so you can eliminate B and D.
Chin charges a rate for 2 hours, then charges a reduced rate for 4 hours. There are no discounts present in his rate, so you can eliminate A.
The equation for Chin's charges can be found by the equation C. 2x + 4y = 84.
Credit. That's why the stock market crash was so terrible in 1929.
To take out terms outside the radical we need to divide the power of the term by the index of the radical; the quotient will be the power of the term outside the radical, and the remainder will be the power of the term inside the radical.
First, lets factor 8:
Now we can divide the power of the term, 3, by the index of the radical 2:

= 1 with a remainder of 1
Next, lets do the same for our second term

:

= 3 with a remainder of 1
Again, lets do the same for our third term

:

with no remainder, so this term will come out completely.
Finally, lets take our terms out of the radical:

We can conclude that the correct answer is the third one.