Answer
$3x^{\frac{1}{2}}+5y^{\frac{3}{4}}$
Work Step by Step
We are given:
$(\sqrt[4] (9x)^2 + \sqrt[4]625y^3$
$=(\sqrt[4] (3^2x)^2+ \sqrt[4] 5^4y^3$
$=\sqrt[4] 3^4x^2+5\sqrt[4]y^3$
$=3\sqrt[4] x^2+5y^{\frac{3}{4}}$
$=3x^{\frac{1}{2}}+5y^{\frac{3}{4}}$