Final answer:
The length of bar QC cannot be determined with the given information; additional context or a diagram is needed to provide a precise answer.
Step-by-step explanation:
To find the length of bar QC when AC = 12 cm and QD = 10 cm, and given the length c = 12 cm, we need more information about the shape and the relationships between the points A, C, Q, and D. Without additional information, for instance whether they form a triangle, rectangle, or any other shape, or if there are certain properties (like parallelogram properties) that we can use, it is impossible to determine the length of QC. Furthermore, the reference information doesn't provide the necessary context to solve for QC. More context or a diagram are needed to give a precise answer.