I'm doing a practice exam for a math test on thursday, wondering if anyone could help figure out how to get from one step to the next. i dont think that the background info is necessary for these two steps. the file is attached (Adobe acrobat). what i am wondering about is the answer under (2b). If the solution for finding (del)u(x,y) were lettered a through d (4 steps, i am wondering how the professer got from b to c. of course i have emailed him as well.