well we know the line will be equal to 180 degrees so we will have an equation like this
x+100+3x=120
we subtract 100 from 180 to get 80.
180-100=80
however x cannot be 80 bc we still have the 3x.
when you do the math, you will see that 20 times 3 is 60.
20×3=60
going even further we see that 60 plus 20 is 80
60+20=80
so to check our work, lets solve this equation
20+100+3×20=180
20+100=120
3×20=60
120+60=180
so your answer is x=20
