In the image, you have 3 parallel lines cut by a transversal and a given angle that measures 102° (let's call it 4).
The theorem of alternate exterior angles says that those angles are congruent. This means they have equal measures. Then, angle 2 is congruent to angle 4 as they are alternate exterior angles (they lie outside the two lines and on opposite sides of the transversal), then angle 2 measures 102°.
Alternate interior angles are those that lie between the two lines and on opposite sides of the transversal, then angle 4 and angle 1 are alternate interior angles, and the theorem says they are congruent, then angle 1 measures 102°.
Angles 2 and 3 are supplementary, which means they add up to 180°, then if angle 2 measures 102°, angle 3 measures 78° (180-102=78)