Question

if every positive number has two square roots and you can find the length of the side of a square window by finding a square root of the area, why is there only one answer for the length of a side

Answer 1

Because even though the math is there, when you go to cut the pieces to build the window frame with, you can't make pieces with a negative length. So if the answer is going to be used in the real world, we use the positive one, and ignore the negative one.