Answer:
The definition of alternate interior angles is that those two angles are the same.
Therefore because they are the same you can set them equal to eachother
3x + 24 = 78 is true
3x = 54
x = 18
Just know the definitions, for instance 3 and 4 are the same, 2 and 5 are the same, 1 and 6 are the same
