The general solution of the differential equation $x \, dy - y \, dx = y^2 \, dx$ is:
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When you see $x \, dy - y \, dx$, try dividing by $y^2$ to get $-d(x/y)$ or by $x^2$ to get $d(y/x)$. Here, dividing by $y^2$ gives $d(x/y) = -dx/x$ if rearranged slightly, which is even faster.