This is actually a relatively simple problem, but because the other angle is missing, it seems complex.
The angle adjacent to the 100 degree outside the triangle is 80 degrees. We know this because it is a supplementary angle, or an angle that, when added along with another angle’s measurement, makes 180 degrees. The line is straight, so the other angle must be 80 degrees to add up with the 100 to make 180 degrees.
Now that we know the other unknown angle, we can use the fact that the sum of angles within a triangle make 180 degrees. The two angles inside the triangle that we know make 150 degrees (80 plus 50), so the final angle ‘x’ must be 30 degrees (180-150=30).