Remember that
The formula to calculate the number of triangles formed by diagonals from one vertex is given by
n-2
where
n is the number of sides
so
Problem 1
14 gon
the value of n=14
14-2=12
therefore
The answer is 12 triangles
N 3
19 gon
n=19
19-2=17
The answer is 17 triangles
N 5
12 gon
n=12
12-2=10
The answer is 10 triangles