\]
(a) To find the joint marginal density function of $Y$ and $Z$, we integrate the joint density function over $X$:
\[f_{Y, Z}(y, z)=\int_{0}^{y} f(x, y, z) d x=\int_{0}^{y} \frac{4}{9} x y z^{2} d x=\frac{4}{9} y z^{2} \int_{0}^{y} x d x=\frac{2}{9} y^{2}
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