There are 16 rods. A square has 4 equal sides, so each side must consist of 4 rods. The length of each rod is 8 cm, so the length of each of the square's sides is 32 cm. Therefore, the area of the square is:
When finding the value of a in your working, I suggest to leave the answer to 5 significant figures so that your final answer is more accurate. The value of a should be 26.048m to 5 s.f.
Then multiplying 26.048 with sin 32° would give you 13.8m too when rounded off to 3 significant figures :)