Peter is making new cushion covers for a chair. There are three sizes of cushions on the chair. What is the surface area of the seat back cushion? 21 in 3 in 3 in 10 in seat back 15 in 10 in arm rest 15 in 21 in seat bottom 18 in 3 in Net for Seat Back 21 in 21 in 3 in 3 in 21 in 15 in​