geometry

because the lines are parallel, we can conclude that angle 3 + angle 5 is 180 (they both make up a straight angle).

 

now we can make this equation:

 

5x + 10 + 3x + 10 = 180

8x + 20 = 180

8x = 160

x = 20

 

hope this helped! sorry if my explanations weren't too clear. feel free to ask any questions you have about what i did! :)