Consider the function of two variables

u(x,y) = f(x-y) + g(x+y/3)

where f(s) and g(t) are each arbitrary functions of a single variable.

Using the change of variables

s=x-y and t=x+y/3

what are the first and second derivatives of u with respect to x and y in terms of f and g??thankssss

(Bow)