We can also compute areas by slicing horizontally instead of vertically. In this case, we'll denote by $W(y)$ the width of a horizontal strip.
The area of each slice is $W(y)\cdot\Delta y$, and the total area becomes $\displaystyle\int_\alpha^\beta W(y)\,dy$, where $\alpha$ and $\beta$ are the smallest and largest values of $y$ (not the smallest and largest values of $x$). To figure out the width as a function of $y$, we often have to rewrite the curves in terms of inverse functions: $x=f^{-1}(y)$, instead of $y=f(x)$.