Let take a rectangle of length 36 and width 13.5 feet.
So calculate the total area of big recatngle and subtract the small are:
[tex]\begin{gathered} A=(36\times13.5)-2(16.5\times1.5) \\ A=486-2(24.75) \\ A=486-49.5 \\ A=436.5ft^2 \end{gathered}[/tex]The monthly cost per square feet is:
[tex]\begin{gathered} C=\frac{825}{436.5} \\ C=1.89 \end{gathered}[/tex]And the length of each side of one of the square represents