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?

As A and B are squares, the 4 sides of A must be equal to each other and the same goes for the 4 sides of B. 15cm -12cm= 3cm will give the length and breadth of square B. While all 4 sides of square A is 12cm each.