Triangular prism

$An$ $edge$ $is$ a line segment between a $3D$ $Figure$

so the number of line segments between a triangular prism is nine :

1st: each of the two triangles have 3 line segments = $6$

2nd: the line connecting them is 1 line segment = $1$

3rd :the rectangle has four , no 2 line segments because 2 of them have been taken by the triangles = $2$

adding them up we get 9