I think
[tex] \dfrac{4}{\sqrt{21}} [/tex]
is about as simple as this is going to get. For some reason teachers like the radicals in the numerator, so we multiply top and bottom by [tex]\sqrt{21}[/tex] and get
[tex] \dfrac{4}{\sqrt{21}} \times \dfrac{\sqrt{21}}{\sqrt{21}} = \dfrac{4\sqrt{21}}{21}[/tex]
Answer: [tex]\dfrac{4\sqrt{21}}{21}[/tex]
[tex] \frac{4}{\sqrt{21}} =\frac{4\sqrt{21}}{\sqrt{21}\sqrt{21}} =\frac{4\sqrt{21}}{21} [/tex]
