Why is the answer not all of them?

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First, find the total area of pizza. The area of a rectangle is length times width. 

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.