This theorem is the alternate exterior angle theorem.
Here is why:
Below I have attached an image of your problem, but I highlighted the inside. This is a trick that makes it very easy to see which type of angles are in problems like these.
The two parallel lines are cut by a transversal, the inside of these lines, which I shaded is called the interior. The outside, where I have not shaded is the exterior.
As you can see, these two angles are alternating across the transversal and on the exterior, so they will be alternate exterior angles.
Alternate exterior angles will be congruent, along with the other angle theorems, except same side interior angles are supplementary.