In any triangle,



So, if all you want is just the Third angle, then
Just add the two given angles 65°+35°, getting 100°.
Then subtract that from 180°.

180° - 100° = 80°

So the third angle is 80°.  That's the answer!

Then the 2(x-10) is not necessary at all.

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However, if your teacher wants you to find x, then 

2(x-10) = 80°
 2x-20° = 80°
   +20°  +20°
 2x     = 100°
  x     = 50°
 
Edwin