#### Answer

$28-21i$

#### Work Step by Step

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}