A contractor needs to buy nails to build a house. The nails come in small boxes and large boxes. Each small box has 200 nails and each large box has 400 nails. The contractor bought 3 times as many large boxes as small boxes, which altogether had 2800 nails. Graphically solve a system of equations in order to determine the number of small boxes purchased, x, and the number of large boxes purchased, y.