If you know the leg lengths of two right triangles. can you tell whether they are congruent? Explain your answer

Answer: the same length for one of the other two legs.

It doesn't matter which leg since the triangles could be rotated. If the hypotenuse and one leg of one right-angled triangle are equal to the corresponding hypotenuse and leg of another right-angled triangle, the two triangles are congruent.