Functions of two variables and partial derivatives practice question 6

With $y$ held fixed, $f$ is a quotient of two functions of $x$. Which rule applies, and in what order does its subtraction go?

Hold $y$ fixed and use the quotient rule in $x$. The top and the bottom each have $x$-derivative 1.

The numerator simplifies: $(1)(x + 2y) - (x - y)(1) = 3y$.