Final answer:
The provided dimensions cannot determine the unknown side of a triangle without additional context or clarification on the type of triangle or geometric setup.
Step-by-step explanation:
The question asks to find the unknown side of a triangle, which suggests the application of the Pythagorean theorem or a similar triangles concept. However, the values provided (4 cm, 9 cm, 3 cm, 19 cm) do not pertain to a specific triangle where a direct formula can be applied. There may be a typo or missing context for this problem, making it impossible to determine the correct side without additional information or clarification. As per the guidelines, a proper geometric context or triangle type (right, obtuse, acute) must be provided to correctly find an unknown side of a triangle.