Let's draw a diagram of this room so it's easier to solve. (see the image I've included)
The perimeter is the length of all of the sides of the rectangle, so
8 + 8 + x + x = 40
Simplify
16 + 2x = 40
Subtract 16 from both sides
2x = 24
Divide by 2 on both sides
x = 12
The length of the rectangle is 12.
Hopefully this helps! If you don't understand or have more questions, feel free to DM me!! :)
