The figure can be considered to be a 25 in by 20 in rectangle with a 5 in by 7 in triangle cut off the corner. Then the area of the figure is the area of the rectangle, less the area of the triangle.
The slope of the line l is the opposite reciprocal of y+2x=9. Rearranged the equation to y=9-2x and we get a slope of -2. The opposite reciprocal of -2 is 1/2.