Subtract 35 from 305, and you get 270. The formula for the area of a triangle, where b is base and h is height, is b * h / 2. There are three faces, or 3 triangles, and we know the base, so we can say that 3 * 9 * h / 2 = 270 => 27 * h / 2 = 270 => 27 * h / 2 * 2 = 270 * 2 => 27 * h = 540 => 27 * h / 27 = 540 / 27 => h = 20, so the slant height is 20 inches.
