Assuming you are focusing on the square only, then the answer is 9 inches.
You take the square root of 81 to get 9. Notice how 9^2 = 9*9 = 81. So if you're not familiar with square roots, then think backwards in a sense to get the answer.
1/3 + 7x = 1/3 + 7(5 )=
1/3+35
=1/3 + 105/3 =
106/3=
35 1/3
Where is the diagram? How do we answer without it?