My Calculus book is just going on about derivations to formulas so I would need help with this question:

An area on a map is provided with rectangular xy-coordinates. The height over the ocean is given by the function h(x,y). If h'x(a,b)=0.1 and h'y(a,b)= -0.2 what could be said about the ground at the point (a,b), ie what is its slope?

I know how to calculate partial derivatives but I dont really understand their purpose or the answer to the above stated question.

Help would be muchos appreciated!