If two roots have the same index, you can multiply their content:
[tex]\sqrt[3]{a}\cdot \sqrt[3]{b}=\sqrt[3]{ab}[/tex]
So, you have
[tex]\sqrt[3]{9x}\cdot\sqrt[3]{49x}=\sqrt[3]{9x\cdot 49x}=\sqrt[3]{441x^2}[/tex]
