Verified Answer :

\(\sqrt{90}=\sqrt{9\times10}\\--------------------\\use\ \sqrt{a\times b}=\sqrt{a}\times\sqrt{b}\\--------------------\\=\sqrt{9}\times\sqrt{10}=3\times\sqrt{10}=\huge\boxed{3\sqrt{10}}\)

TSO

\(\sqrt{90} = \sqrt{9 \times 10} = \sqrt{9} \times \sqrt{10} = 3 \times \sqrt{10} =3 \sqrt{10}\)

Using thisĀ formula:
\(\sqrt{a \times b} = \sqrt{a} \times \sqrt{b}\)

Hope that helps :)