Area = 21 x 36 = 756 sq. inches
Now, we must divide the total area with the largest square possible without any excess. So, we equate the area of the pizza to area of square:
A = s² = 756
s = 6 √21
Hence, the largest piece square possible has a side of 6 inches. Then, we divide the total area of the pizza by the area of each square piece to find the number of pieces;
756/6² = 21
Thus, there would be 21 pieces of 6-in square piece of pizza.