Please confirm that I understand the question. The cases are 26.91 ft² and you need to get enough cases to cover 600 ft². What is the 17x17?
If the 17x17 is just the name of the cases, then you will need 600 / 26.91 = 22.297 cases. If you can break up the cases, then you will need to buy 23 of them to cover the 600 ft²
If I am not understanding the problem properly, let me know and I will recalculate.