I believe what you are looking for here is an expression. For this example, the cost of a pen will be represented by x. We know that a folder costs $1.15 more than a pen, so the cost of a folder will be represented by x+1.15. This question is asking for 6 pens, and so we multiply the cost of a pen (x) by 6 and get 6x. It also asks for 3 folders, and so we multiply the cost of a folder (x+1.15) by 3 and get 3(x+1.15). When you simplify that by multiplying the 3 by the x and then the 3 by the 1.15, you get 3x + 3.45. To get the total cost of 6 pens and 3 folders, add the two together for an answer of 6x + 3x+3.45. Simplify that by adding the two x's together and get 9x+3.45, where x is the cost of a pen.
