Wyatt's room is a rectangle with a perimeter of 40 feet. the width of the room is 8 feet. what is the length

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!! :)