The graph below shows the price of different numbers of mats at a store: A graph is shown. The values on the x axis are 0, 2, 4, 6, 8, 10. The values on the y axis are 0, 21, 42, 63, 84, and 105. Points are shown on ordered pairs 0, 0 and 2, 21 and 4, 42 and 6, 63 and 8, 84. These points are connected by a line. The label on the x axis is Number of Mats. The title on the y axis is Price in dollars.

Which equation can be used to determine p, the cost of b mats?