The problem wants us to complete a sentence istarting with " If ....
The first blank should have a condition between the two segments shown on the right for them to be congruent.
The answer is:
If AB = CD (meaning the distance between points A and B equal the distance between points C and D). Notice that in the first part they used AB to symbolize the distance between points A and B (or the length of the segment).
Then, if the length of the segments are equal, the segments are congruent, and we write the congruence with a symbol that looks like an equal sign with a little wiggle on top, like the one shown below.
[tex]\cong[/tex]
So in the last blank for the sentence we use:
[tex]AB\cong CD[/tex]
