Based on your diagram in the picture, the length of the side of A will equal to the length of a side of B plus the longer length of C which is equal to 12cm. Similarly, the length of side of A and B will equal to 15cm. This will make the length of side of B equal to 3cm which is the shorter breadth of C. With the length of side of B known, we can calculate the length of C to be 9cm. The area of C will be equal to the product of the length and breadth of C. Just curious, do you need to solve this by algebra?