Hello from MrBillDoesMath!
Answer:
Either length of hypotenuse or lengths of other leg.
Discussion:
"HL" stands for "hypotenuse-leg". If 2 right triangles satisfy the HL postulate, then they are congruent. From the diagram we need to be told either the length of the hypotenuse or of the other leg (which can be used with Pythagoras to determine the hypotenuse)
Thank you,
MrB