A regular nonagon has 9 equal sides each of which subtends an angle of 360/9=40° at the centre. The interior angles each measure 180-40=140°. The exterior angles measure 40°.
So 9x+5=140, 9x=135, x=15°; 11x-25=140, 11x=165, x=15°.
We can also write 9x+5=11x-25,
30=2x, x=15°. So all this calculations consistently confirm that x=15°.