Okay, so my problem lies within taking the second derivative of a change of variable equation. w = f(x,y); x = u + v, y = u - v so far I have the first derivative: dw/dx = (dw/dv)(dv/dx) + (dw/du)(du/dx) = (d/dv + d/du)w Now I'm having problems in finding my second derivative: d2w/dx2 = ? I don't necessarily want the answer, but more of how to get to the answer. I figure if I get how to do it with dw/dx, dw/dy would follow suit. Any help would be greatly appreciated! Thanks.