7√21 × 2√2 × √42

First, let's simplify each term under the square roots:

√21 = √(3 7) = √3 √7
√2 = √2
√42 = √(6 7) = √6 √7

Now let's substitute these values back into the original expression:

7(√3 √7) × 2√2 × (√6 √7)

Now, multiply the numbers outside the square roots:

7 * 2 = 14

And multiply the numbers inside the square roots:

√3 √7 √6 √7 √2

Since we have pairs of the same numbers inside square roots, we can simplify the expression:

14 3 7 6 2 = 1764

So, 7√21 × 2√2 × √42 simplifies to 1764.