If we draw diagonals from a fixed vertex of a n sided polygon, there will be (n-2) number of triangles formed.
So for n=11, number of triangles formed = 9
for n=12, number of triangles formed = 10
for n=13, number of triangles formed = 11
for n=14, number of triangles formed = 12
for n=15, number of triangles formed = 13