Answer:
4
Step-by-step explanation:
4 is the greatest commen factor of 20 and 84
4 x 5 = 20
4 x 21 = 84

The required additional information required to prove the Triangles congruent are :

The Triangles will be congruent by HL congruency.

The Triangles are congruent by LL congruency.

The Triangles are congruent by HA congruency.

The Triangles are congruent by LA congruency.
Try 3 because 3 times 3 is. 9