Good question! For a rectangle, it is in the middle and for a triangular load, the resultant force is acting 1/3 of the base from the right angle of the triangle.
Using similarities of triangles, before it was cut the bigger triangle had 200N/m, after it was cut the smaller triangle was exactly half of the bigger one, finding the ratio(i.e 200:x=6:3) , x= 100. Note that 6 and 3 are just the base distances of a bigger and smaller triangles respectively.
