How do you simplify -5sqrt21*-3sqrt42?

Apr 10, 2015

You can write your product as:
$\left[- 5 \sqrt{7 \cdot 3}\right] \cdot \left[- 3 \sqrt{7 \cdot 6}\right] =$

$= \left[- 5 \sqrt{7} \sqrt{3}\right] \cdot \left[- 3 \sqrt{7} \sqrt{6}\right] =$

$= \left[- 5 \sqrt{7} \sqrt{3}\right] \cdot \left[- 3 \sqrt{7} \sqrt{3} \sqrt{2}\right] =$

Multiplying:

$= 15 \cdot 7 \cdot 3 \cdot \sqrt{2} = 315 \sqrt{2}$