A three-row panel
The first row of the panel is 2, 7, 6. The second is 9, 5, 1. The third is 4, 3, and a blank. The panel uses the numbers 1 through 9 exactly once each.
Keep the total consistent
Every row and every column sums to 15. Find the missing number. You can look for the unused number or calculate from the third row or column. The approaches should agree.
Check rather than guess
Once you have a candidate, place it back in the panel and check its row and column. Keep the checking step rather than reporting a number simply because it seems plausible.