#### Answer

$-21$

#### Work Step by Step

The product of two numbers with the different signs can be found by performing the following steps:
(1) Multiply their absolute values.
$|-3| \cdot |7| = 3 \cdot 7 = 21$
(2) The product is negative.
$-3(7) = -21$
Thus, the product of the given numbers is $-21$.