Verified answer

= - sqrt(21) / - 2(sqrt21)^2

= sqrt(21) / 2*21

= sqrt(21) / 42

√21/√21 = 1

-1/-2 = 1/2

so we have..

1/2√21 and we will multiplly this by √21/√21 to rationalise the denominator...

1/2√21 * √21/√21

=√21/(2*21)

=√21/42