Thank you for the help on the last problem, but for this one I'm still stumped as to what I need to do, even after reading a bit on wikipedia. Any help would be much appreciated.
But, I don't know what u and are.
In Cartesian coordinates in three dimensions, .
Here, you don't have any dependence on y or z so your equation reduces to
"u" is function. You are asked to replace u by in that equation and see if it is true.
But what course did you see this in? I cannot imagine you being asked to do a problem like this if you were not already expected to know what was!