Well, I'm not sure I understand it myself!
The first part says x is a member of D(f), though we're not told what D(f) means. I guessed it means the Domain of a function f.
The backward, upper case E, means "there exists", so I simply guessed the whole thing means: if x is the set of variables in the domain of f there exists a specific value of x such that f(x) equals 6.
Still doesn't make a lot of sense though!!