Chapter 5 - Section 5.6 - Complex Zeros; Fundamental Theorem of Algebra - 5.6 Assess Your Understanding - Page 394: 4

$3-4i$

The Conjugate Pairs Theorem states that if $a+bi$ is a zero of a polynomial function with a real number coefficients, then $a-bi$ is also a zero of the function. Thus, if $3+4i$ is a zero of a polynomial function with real coefficients, then $3-4i$ is also a zero of the function.