The sum of exterior angles is always 360, so to solve this one, you will have to add the exterior angles all up to find x:
10x + (4x + 26) + (12x + 44) + (7x - 3) + (6x + 59) = 360
39x + 126 = 360
39x = 234
x = 6
Now, the exterior angle at vertex C is 4x + 26.
Plug in the value you got for x:
4 * (6) + 26 = 50
The answer is 50 degrees.