Chapter 7 - Section 7.7 - Complex Numbers - Concept and Vocabulary Check - Page 569: 9

$-4i$

To divide two complex numbers, multiply the numerator and the denominator by the conjugate of the denominator. The denominator is $4i$, which can be written as $0+4i$. The conjuagte of this complex number is $0-4i$ or $-4i$. Thus, the missing expression in the given statement the conjugate of the denominator which is: $-4i$