Answer:
9 cm
Explanation:
A square is a 4-sided shape with side lengths of equal measure.
The area of a square can be calculated by squaring the measure of the side length.
![\boxed{\begin{minipage}{4 cm}\underline{Area of a square}\\\\$A=s^2$\\\\where:\\ \phantom{ww}$\bullet$ $A$ is the area. \\ \phantom{ww}$\bullet$ $s$ is the side length.\\\end{minipage}}](https://img.qammunity.org/2023/formulas/mathematics/college/ztj8tk4g8hv9i0knpkc3v3ee4kflvnkz3w.png)
Given the area of the square painting it 81 cm², to find the length of one side, substitute the area into the formula and solve for s:
![\begin{aligned}A&=s^2\\\implies 81&=s^2\\√(81)&=√(s^2)\\9&=s\end{aligned}](https://img.qammunity.org/2023/formulas/mathematics/college/xch021ho690h7qf933bkub0jt18wytyazq.png)
Therefore, the length of one side is 9 cm.