Question 325506
The length of the original square was 6 km. Here is why: 

6+3 = 9

9 squared = 81

You can also solve it algebraically. The area of the square is 81. Let "x" be the original length. If "x+3" is the new length, then "x+3" squared would be equal to 81. "x+3" squared is x^2 + 6x + 9 = 81

subtract nine from both sides to get: 72 = x^2 + 6x

use the quadratic formula, and you will get "6" as your solution.