#### Answer

$315 \sqrt 2$

#### Work Step by Step

$(-5 \sqrt 21) \times (-3 \sqrt 42) $
We multiply the constants 5 and 3. We multiply the numbers inside the square root.
$(-5)(-3) (\sqrt (21 \times 42))$
$15 \sqrt 882$
882 has the factors of $49 \times 18$.
$15 \sqrt (49 \times 18)$
49 is a perfect square. Square root of 49 is 7. Because 7 x 7 = 49
$(15)(7) \sqrt 18$
18 has the factors of $9 \times 2$.
$105 \sqrt (9 \times 2)$
9 is a perfect square. Square root of 9 is 3. Because 3 x 3 = 9
$(105)(3) \sqrt 2$
$315 \sqrt 2$