check the picture below.
recall that angles that lie on a flat-line, namely linear angles, sum up to 180°, so the two siblings lying on that red line, is 5x and 180 - 5x.
180 - 5x, being the exterior angle at the vertex C, stemming from AC.
so, we have three exterior angles, since it's a triangle, 8x, 9x and (180 - 5x), let's bear in mind that the sum of all exterior angles is 360°.
[tex]\bf 8x+9x+(180-5x)=360\implies 12x+180=360\implies 12x=180 \\\\\\ x=\cfrac{180}{12}\implies x=15[/tex]
