I THINK this is what you are describing...to find the area, you will need the diameters A and B
A = sqrt( (9+2.5)^2 + 6^2) = 12.971 yd
Now to find B : 9^2 = (1/2(12.97))^2 + (1/2B)^2 results in B=12.48 yd
Area of field =1/2 A * B = 12.97 * 12.48 = 161.87/2= 80.94 sq yd
80.94 yd^2 / 15 yd^2/bag = 5.39 =~~6 bags required