The set of first coordinates is {2,4,6}, that its domain

The set of second coordinates is {2}, that its range.

The first coordinates are all different, so it's a function.
(It doesn't matter that the second coordinates are not different).

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Given f(x) = x² – 2x, find f(4).

Substitute 4 for x

      f(x) = x² – 2x
      f(4) = 4² – 2·4
      f(4) = 16 – 8
      f(4) = 8

Edwin