This is another problem than I've been stuck on for a long time and I tried reading and watching videos but I only find first order partial differentiation with more than two variables or higher order partial differentiation with only two variables. (I'm not calling f a variable but I am calling x and y variables even though they are functions of other variables.) I am stuck very close to the beginning and am attaching my work as stuck.jpg and have the solution to the problem (which is uploaded as solution.jpg) but I do not understand how to get to u_(ss). Could someone show me how to apply the tree diagram to that? Any input would be GREATLY appreciated! Thanks in advance!