$($b$)$ If the joint pdf of $(x,Y)$ is given by $f_{xY}(x,y)=\{\begin{array}{ccc}x+y\text{;},& 0x1,& 0y1\\ 0& \text{o}\text{t}\text{h}\text{e}\text{r}\text{w}\text{i}\text{s}\text{e}& \end{array}.$ Find the pdf of $Z=xY.$