Question 954475
Not being a finished answer, this may help:


Your 10-sided polygon, regular, will contain 8 triangles, each work 180 degrees, and has 10 vertices.  EACH vertex on the interior has  {{{180*8/10=144}}} degrees.


Your other, outer triangle AXB, is an isosceles triangle.  ...
Can you make the drawing of all this, and find the two equal interior angles of the ISOSCELES TRIANGLE, AXB?