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We will start with the numerator of the square root. 

 

For the first fraction($\large{5 \over {\sqrt {20}}}$), we can rationalize the denominator, by multiplying the fraction by $\large{\sqrt {20 } \over {\sqrt{20}}}$. This gives us $\large{{5 \sqrt {20 }} \over 20}$

We can rewrite $\sqrt{20}$ as $2 \sqrt 5$. This means we have $\large{5 \times 2 \sqrt 5 \over 20}$, which simplifies to $\large{\sqrt 5 \over 2}$

Repeating this process to the second fraction ($\large{\sqrt{845} \over 9}$) gives us $\large{{13 \sqrt 5} \over 9}$. 

Adding all the terms up gives us: $\large{{{26 \sqrt5}\over 18} + {18\sqrt5 \over 18} + {9 \sqrt 5 \over 18}} = {53 \sqrt 5 \over 18}$.

This means that we have: $\large{\sqrt{{53 \sqrt5 \over 18} \over \sqrt 5}}$. 

Now, we just have to cancel out the common terms, and we have our answer. 

 

Can you take it from here?