12. The perimeter of a rectangle is 2*length +2*width. So, in this case (when L is the length of the rectangle) the perimeter should be 2L+2(L-3). If you simplify you will get 4L-6.
13. The perimeter is just the value you get when all the sides of a shape are added. A square has 4 equal sides. So, just divide the perimeter, which in this case is (24x+36), by 4. You should get 6x+9.