Solution 1 :
We calculate the third angle of the triangle (not x) : 180 - 80 = 100
(Note : the angle is supplementary to the 80 degree angle, so the sum of them is 180)
Now we calculate x, and since we know 3 angles in a triangle add up to 180 :
180 - 30 - 100 = 50.
Solution 2 :
If there is an angle that is outside the triangle but is supplementary to one of the angle in the triangle, it will be equal to the sum of the 2 angles which are NOT supplementary to it.
Using this, we can say 80 is equal to 30 + x (because those 2 angles are not supplementary)
So, we can find x by taking 80-30 = 50.