Chapter 10 - Exponents and Radicals - 10.8 The Complex Numbers - 10.8 Exercise Set - Page 686: 47

$28-21i$

Using $a(b+c)=ab+ac$, or the Distributive Property, and the properties of radicals, the given expression, $ -7i(3+4i) ,$ simplifies to \begin{array}{l}\require{cancel} -7i(3)-7i(4i) \\\\= -21i-28i^2 \\\\= -21i-28(-1) \\\\= -21i+28 \\\\= 28-21i .\end{array}