Why is this arithmetic combinator not working?

You have connected the wire from the constant combinator to the output side of the arithmetic combinator; you need to connect it to the input side instead (where the inward-pointing yellow arrow is). Which side the connection is made on depends on which half of the combinator's 2×1 shape you click on when connecting a wire.

How I can tell:

• The small electric pole you've highlighted, which definitely is connected to the output, is showing the input signals, indicating they're sharing a wire. When correctly connected, the input signals will not appear there (unless you run another red or green wire to the constant combinator).

• The line of the wire from the constant combinator is heading more towards the middle than the red input post of the arithmetic combinator.

  (Wiring is one of the rare places where it can be useful to view them with alt mode off, so the symbols are not obscuring the physical structures.)