The property of complex numbers demonstrated by the equation \((5 \times i) \times (9x) = 5 \times (i \times 9x)\) is the Associative Property of Multiplication.
The associative property states that the way in which numbers are grouped in multiplication does not affect the final product. Formally, for any numbers \(a\), \(b\), and \(c\), the associative property can be expressed as:
\[ (a \times b) \times c = a \times (b \times c) \]
In your case, by showing that \((5 \times i) \times (9x) = 5 \times (i \times 9x)\), you are verifying that the grouping of the factors does not change the result, thus confirming the associative property holds true for complex numbers just as it does for real numbers.