Two of the exterior angles of a triangle are 158 degrees and 99 degrees. Find the third exterior angle, in degrees.

If you take one exterior angle at each vertex of any polygon (triangle, quadrilateral, ...), their sum will always 360^{o}.

So, the sum of 158^{o}, 99^{o}, and the unknown angle is 360^{o}.

---> 158 + 99 + x = 360 ... solve for x ...