Pedro's father is building a shed, how many square feet of wood are needed to build the back of the shed

That would depend on the length and the height

Do you have a picture or any more information? You need to determine the area of the wall of the back of the shed. 
If it is a square, then you simply need to multiply the two sides a x b; if there is a triangle on top of that then you need to add the area of the triangle (h*b)/2. 
Hope this helps.