Discrete structure question

Here is a picture of four models. Some of the cubes are hidden behind other cubes. Model one consists of one cube. Model two consists of four cubes and so on. How many cubes are in the third model? ii. How many cubes are in the fourth model? If a fifth model were built, how many cubes would it take? iv. Find an expression for the number of cubes used in the nth model. V. Sketch a side view, front view and plan view of the fourth model