Question: How to find total derivative by using total derivative operator?

I have two functions u(x,y,t) and p(x,y,t).

I want to find the total derivative of D_x( p*u) (where * is the multiplication)

where D_x is as given in the attached file formula_of_D_x.pdf 

Also, it is given in the following link but it's not clear to me. I think it somehow resembles my query, but not clear about the syntax so that I should implement it on mine.

Please Wait...