First calculate the area of the garden, using the formula for area of a triangle: A=(1/2)bh. For a right-angle triangle the length of the base (b) and height (h) are the same as the two legs. Divide the area by 12 to find how many bags are needed for 2" deep mulch. Then multiply that by 3/2 so that the depth is increased from 2 to 3 inches. If you end up with an answer that is not a whole number round up to the next whole number. This should give the answer of 17.