Geometry

Question Analysis
This question involves applying the Hypotenuse - Leg (HL) Theorem and the Pythagorean Theorem.
The main focus is to use the fact that congruent right - triangles have equal side lengths and then find the missing side of one of the triangles using the Pythagorean Theorem.

Key Concept Explanation
The Hypotenuse - Leg (HL) Theorem states that if the hypotenuse and a leg of one right - triangle are congruent to the hypotenuse and a leg of another right - triangle, the two right - triangles are congruent.
The Pythagorean Theorem for a right - triangle with legs and and hypotenuse is .

Step - by - Step Solution
1. In right - triangle , we know that cm (hypotenuse) and cm (leg). Let the other leg be .
  - According to the Pythagorean Theorem .
  - Substitute and into the formula: