The answer is "that supplements of congruent angles are congruent" because in order for the angles to be congruent to complete SAS their supplements must be congruent.
From the question, we know that we will be looking at <3, <4, and angles TKL and TLK. That being said, since <3 is congruent to <4, that means that angles TKL and TLK, which are each supplementary to either angle 3 or 4, are congruent because, since angles 3 and 4 are congruent, they are congruent because the supplements of congruent angles are congruent.
