If the shape is a rectangle, your formula for perimeter is 2L + 2W. Given that our perimeter is 40 and our length is 12, we can find the width. Here's how:
Multiply 12 by 2, since our formula is 2L + 2W.
Now we have 24, so subtract 24 from 40 and then divide by 2 to get our width.
40 - 24 = 16.
16/2 = 8.
Our width is 8in.
Now that we have our width, we can find the area. The formula for area is L x W.
Our length is 12, and our width is 8.
8 x 12 = 96.
Your area is 96 in^2.
I hope this helps!