From the property of line.
The sum of all the angles around a point on a staright line is always 180 degrees
In the given figure
The angle x , 100º and x are making angle at a single point on the line
So:
x + 100 + x = 180
Add similar terms together :
x + x + 100 = 180
2x + 100 = 180
Subtract 100 from both side :
2x + 100 - 100 = 180 - 100
2x = 80
Divide both side by 2
2x/2 = 80/2
x = 40º
Answer : x = 40º