Define the function: z = f(e, y) = ev. Consider the contour z = c, where c is a positive real number. 1. Solve for y as a function of a along the z = c contour and calculate dy/dx and d'y/ dx. 2. Treat f(x, y) = c as defining y(x) implicitly and use the Chain Rule for functions of 2 variables to calculate dy/dr and d'y/da