How many ________does it take...

[Obi2Kenobi]

Have you had calculus?  If you haven't, d/dx is a calculus symbol for the derivative of something in terms of x.  The d's and the x aren't an extras variables.  d/dx x^2 means take the derivative of x squared.
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That would be my confusion. :) *curses at notation* That's the one major problem with learning most of my math outside of school.