You may be thinking of non-overlapping triangles in the picture with 20 dots.
That's a different situation.
Imagine the dots are placed apart from left to right, not on a straight line.
Pick the leftmost point.
To make a triangle, you can pick
any 2 of the remaining 19 dots.
The number of ways to do this is
Therefore, there are 171 triangles that can be drawn which include the leftmost point.
If you move on to the next point to the right and exclude the point previously chosen,
then you can draw another
triangles.
These triangles do not include any of the previous 171,
since these 153 omit the leftmost point.
Notice that these triangles typically share sides of other triangles,
but they are made of 3 distinct points, hence the triangles are being counted only once.
hence, there are
triangles.