it depends on what you mean by a diagonal. If you take a diagonal to mean a line from one corner to the "opposite" corner, then a nonagon doesn't have any, and a decagon has five. The reason a nonagon doesn't have any, is that the corners aren't opposite each other.
A decagon has ten vertices.
A decagon has 10 sides
A decagon has 10 angles (and sides)
There are 8 triangles inside a 10 sided decagon
There are 35 diagonals in a 10 sided decagon
A decagon has 7 more sides than a triangle. A triangle has 3 and a decagon has 10.
12
10
35.
35
A tetra-decagon is a 14 sided polygon having 77 diagonals