The area of a square is 81 feet. What is the perimeter?

To find the perimeter of a square, you add the lengths of the sides togother: $${\mathtt{p}} = {\mathtt{s}}{\mathtt{\,\small\textbf+\,}}{\mathtt{s}}{\mathtt{\,\small\textbf+\,}}{\mathtt{s}}{\mathtt{\,\small\textbf+\,}}{\mathtt{s}}$$;  $${\mathtt{p}} = {\mathtt{perimeter}}$$; $${\mathtt{s}} = {\mathtt{side}}$$

Since you do not know the the lengths of the sides but you do know the area, you find the the length of one side by using the area formula. 

$${\mathtt{a}} = {{\mathtt{b}}}^{{\mathtt{2}}}$$;  $${\mathtt{a}} = {\mathtt{area}}$$; $${\mathtt{b}} = {\mathtt{base}}$$; 

$${\mathtt{81}} = {{\mathtt{b}}}^{{\mathtt{2}}}$$

$${\sqrt{{\mathtt{81}}}} = {\sqrt{{{\mathtt{b}}}^{{\mathtt{2}}}}}$$

$${\mathtt{b}} = {\mathtt{9}}$$

Now that you know that one of the sides is 9 feet and that all sides of a square are the same length, you can now use the perimeter formula $${\mathtt{p}} = {\mathtt{s}}{\mathtt{\,\small\textbf+\,}}{\mathtt{s}}{\mathtt{\,\small\textbf+\,}}{\mathtt{s}}{\mathtt{\,\small\textbf+\,}}{\mathtt{s}}$$

$${\mathtt{p}} = {\mathtt{9}}{\mathtt{\,\small\textbf+\,}}{\mathtt{9}}{\mathtt{\,\small\textbf+\,}}{\mathtt{9}}{\mathtt{\,\small\textbf+\,}}{\mathtt{9}}$$

$${\mathtt{p}} = {\mathtt{18}}{\mathtt{\,\small\textbf+\,}}{\mathtt{9}}{\mathtt{\,\small\textbf+\,}}{\mathtt{9}}$$

$${\mathtt{p}} = {\mathtt{27}}{\mathtt{\,\small\textbf+\,}}{\mathtt{9}}$$

$${\mathtt{p}} = {\mathtt{36}}$$

The preimeter of a square that has an area of 81 feet is 36 feet.

Very nice answer and presentation, gibsonj338.....!!!