Question

How do you simplify `4/sqrt21`? Thank you very much for your help.?

Answer 1

I don't know if it is much of a simplification but you could rationalize the denominator by multiplying the top and bottom by `sqrt(21)`

`4/sqrt(21) = (4sqrt(21))/(21)`

As I said, not much of an improvement, but some people prefer to keep roots out of the denominator.
There is no real simplification beyond this.

Answer 2

I would multiply and divide by `sqrt(21)` and get:
`4/sqrt(21)*sqrt(21)/sqrt(21)=4sqrt(21)/21=sqrt(336)/21`