The Chain Rule & Implicit Differentiation · Hard problem

A hiking trail stays at constant elevation on a hillside whose height is given by $E(x,y) = x^3 - 3xy^2$ (in meters, with $x,y$ in kilometers of east/north position). At the point $(2,1)$ on the trail, the hiker's $x$-coordinate is increasing at $0.5$ km/h. Find the hiker's speed (in km/h) at that instant.