We are given the following information
Length of rectangular deck = 12 feet
Perimeter of rectangular deck = 36 feet
Recall that the perimeter of rectangular shape is given by
![P=2l+2w_{}](https://img.qammunity.org/2023/formulas/mathematics/college/itmgbk10oj9qu3u0xmaq0f8tl00bct4taz.png)
Where l is the length, w is the width, and P is the perimeter of the rectangle.
Let us substitute the given values and solve for width (w)
![\begin{gathered} P=2l+2w_{} \\ 36=2(12)+2w_{} \\ 36=24+2w_{} \\ 36-24=2w_{} \\ 12=2w_{} \\ (12)/(2)=w \\ 6=w \\ w=6\: feet \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/kr90d4led3yayex7me22ogdl376dktnxm3.png)
Therefore, the deck's width is 6 feet
Recall that the area of a rectangular shape is given by
![A=l\cdot w](https://img.qammunity.org/2023/formulas/mathematics/high-school/czdrip2qi4fvur9c8tlj02ezx7mv0rha9f.png)
Where l is the length and w is the width of the rectangle.
![\begin{gathered} A=l\cdot w \\ A=12\cdot6 \\ A=72\: ft^2 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/osmcbs43255fp6mmudh6pvxtfzl3qqodws.png)
Therefore, the deck's area is 72 square feet.