Factor $28=2^{2}\times 7$. Rewrite the square root of the product $\sqrt{2^{2}\times 7}$ as the product of square roots $\sqrt{2^{2}}\sqrt{7}$. Take the square root of $2^{2}$.
$$\frac{\sqrt{7}}{2\sqrt{7}}$$
Rationalize the denominator of $\frac{\sqrt{7}}{2\sqrt{7}}$ by multiplying numerator and denominator by $\sqrt{7}$.
