Where are you stuck? You know how to find the area of a rectangular region. You've multiplied to find the area of each wall, and added these areas together. You've multiplied to find the areas of the windows and doors, and subtracted these from the total wall area. You've divided the area to be painted by the area covered by one can to find the number of cans. You've multiplied by the per-can cost to get the total cost. And... then what? What is your question?

Please be complete. Thank you!