Please help as fast as you can. Thanks.

First, we should find out the formula for the area of an octagon (if you already know this Guest, just scroll down).

This is the formula:

$A=2(1+\sqrt2)a ^2$

(where a is the length of a side)

Since we know that the length is 12, we can easily replace the a for 12.

$A=2(1+\sqrt2)12 ^2$

If you are lazy like me, you will dutifully plug it into a calculator and achieve the approximate result of 

$695.29$ $cm ^2$

The value rounded to five decimal places is 

$695.29351$ $cm ^2$

Hope this helped!