1. The problem statement, all variables and given/known data 2. Relevant equations The product of the slopes has to be equal to -1. 3. The attempt at a solution Well, as the function, call it u, is equal to a constant, the derivative of u with respect to x is the partial derivative with respect to x + (the partial derivative of u with respect to y * dy/dx) = 0 So dy/dx = - (∂u/∂x) / (∂u/∂y) But from here on I don't know how to continue. Could anyone provide a hint?