Answer:
In order of the boxes given:
C
E
A
F
B
D.
Explanation:
That first line is given C.
Alternate interior angles are the angles inside the parallel lines on opposite sides of the transversal at the different intersections.
Alternate interior angles are congruent if the lines are parallel (I know I included this in my "definition"; but the lines don't have to be parallel for these angles to be alternate interior). However, if the lines are parallel which they are here you can say that the alternate interior angles are congruent by the Alternate Interior Angles Theorem.
So the next box is E.
If a pair of angles is congruent, then their measures are equal. So the third box is A.
Angle CAD and angle 4 lay have a common vertex on a straight line so their measure together should be 180 degrees.
These are called a linear pair. Linear pairs are supplementary and adjacent.
So The fourth box goes with F.
They wrote that Angle 3+Angle 5=Angle CAD because if you put those two angles together they will form the Angle CAD.
This Angle Addition Postulate. So the fifth box goes to B.
The sixth line they substituted equation 5 into 4. I noticed I use the word substituted so this will goes with D.