The perimeter is the sum of all three sides. So we add 9x-8w, 2x+8y and 4y+4w. We have 3 different variables, but we don't need to know what they mean to answer the question. Think of them as apples, pears and oranges. How many x's do we have (call them apples)? 11x. How many y's (call them pears)? 12y. How many w's (call them oranges)? -4w. (We had 4 oranges but we owed someone 8 oranges so we still owe them 4!). So the answer is 11x+12y-4w.