Kiana wants to cover an area of her backyard with concrete patio stones in the shape of a right triangle. Each stone costs $0.42 and covers 29.26 square inches. Each square in the grid represents 1 square foot. If (1,1), (7,9), and (15,3), what is the minimum number of stones that Kiana should buy to cover the right triangular area in her backyard? Show your work below.