Please help. This pertains to radical exponents (Algebra 2)

What is the simplest form of:

[tex](9x\sqrt[4]{y})^{3/2}[/tex]


I cannot seem to find the answer. The actual answer is [tex]27x\sqrt[8]{x^{4}y^3 }[/tex] but I don't understand how to get that answer. Can you please help me? Maybe write out how you actually get it? I need help