Cross-sections

https://preview.redd.it/6aoz1er2m8ud1.png?width=880&format=png&auto=webp&s=b125823bb53e9ee99eb85e971a5613a107daa382

I dont even know how to begin. I know i have to identify the xyz plane then somehow i get this equation of y=b-(b/a)x i dont know whats happening??? im so lost