The area of a square is given by the formula:
[tex]A=l^2[/tex]Where l is the length of its side. If the area is given, then the length can be calculated from that formula taking the square root to both sides of the equation. Then:
[tex]l=\sqrt[]{A}[/tex]Substitute A=225 to find the length of the square:
[tex]\begin{gathered} l=\sqrt[]{225} \\ \Rightarrow l=15 \end{gathered}[/tex]