In what horizontal direction should you moce in order to

Question

Suppose that you are standing at the point with coordinates (-100,-100,430) on a hill that has the shape of the graph

z = 500 - (0.003)x²-(0.004)y²

(in units of meters). In what (horizontal) direction should you moce in order to maintain a constant altitude - that is, to niether climb nor descend the hill?