I think the answer is A. Cause your supposed to subtract so 7n-4n=3n, and 3-2=1 :) I hope this helps
Answer: 
This is the same as saying 
====================================================
Work Shown:

where 
As the steps show above, the idea is to factor the radicand into smaller pieces where one of those pieces is the largest perfect square possible. In this case, 36 is the largest factor of 180 that's a perfect square. Then I used the rule
to break up the root.
The parenthesis used at the very end is to help separate the
from the
term. The "i" is not under the square root.
4) Around 1.4
6) Around 5.7
8) Around 4.5
10) Around 9.5
6.5 meters is the answer, i know because i answered the same question on a quiz, i just guessed so i don't really have an explanation on why.