Hello!
If we want our lines to be parallel, they must equal 180°, as they are supplementary measures. Let's set this up as an equation and solve for x.
10x-40+50=180
We subtract 50 from both sides.
10x-40=130
We add 40 to both sides.
10x=170
We divide both sides by 10.
x=17
Therefore, our answer is x=17.
I hope this helps!