Seth's array of six rows and nine columns has a total of \( 6 \times 9 = 54 \) items. Mary's array of nine rows and six columns has a total of \( 9 \times 6 = 54 \) items as well.
The arrays have the same number of items because of the commutative property of multiplication, which states that \( a \times b = b \times a \). Thus, \( 6 \times 9 = 9 \times 6 \).
So, the complete statement would be: The arrays have the same number of items because of the commutative property.