Formula Help - Look up Value for calculation

I trying to look up a value in one table to be used for a calculation in another. Simple stuff I know, but I am struggling and needs an example.

You can access the info from another table by using . so in this case I’m taking the score value from Market 1, Market 2, Market 3

thisRow.[Market 1].Score +
  thisRow.[Market 2].Score +
  thisRow.[Market 3].Score