 If I want to find x to ensure that those two lines L and M are parallel, I would want them to be alternate exterior angles that are congruent. So I would set them equal. I would set 3x plus 10. Set that equal to 4x minus 10. Now the reason that I want to ensure that those two angles are congruent is there is a theorem that says if alternate exterior angles are congruent, then my lines are parallel. And so if I find a value of x that satisfies the hypothesis of that statement, then the law of detachment will say the lines must be parallel. So let's solve this for x. Let's, well, let's subtract 3x first. That leaves me with 10 is equal to, well, 4 minus x minus 10. And then add 10 to both sides. Leaves me with the value of x is 20. And so if x is 20, then my alternate exterior angles will be congruent, and so the lines will therefore be parallel.