Ally has a square backyard with an area of 169 square feet.
Given a square of side length, s
[tex]\text{Area of a Square}=s^2[/tex]Therefore:
[tex]\begin{gathered} s^2=169\text{ square feet} \\ \text{Take the square root of both sides} \\ \sqrt{s^2}=\sqrt{169} \\ s=13\text{ feet} \end{gathered}[/tex]Therefore, the length of one side of the backyard is 13 feet.