Answer :

[tex]5\sqrt[]{-36}+7\sqrt[]{-9}[/tex]

write the numbers as imaginary numbers

[tex]5\sqrt[]{36}i+7\sqrt[]{9}i[/tex]

simplify the roots

[tex]\begin{gathered} 5\cdot6\cdot i+7\cdot3\cdot i \\ 30i+21i \\ 51i \end{gathered}[/tex]