The easiest thing is to copy and paste; the change from c3:al3 to c4:al4, etc, will happen automatically. I would proceed as follows:

1. Enter the formula =average(c3:al3) in AN3.
2. Select cell AN3, press Ctrl-C (or Cmd-C on Mac) to copy it (the cell itself, not the formula from the formula line).
3. Select the range AN3:AN562 (or whichever range you want to paste to).
4. Press Ctrl-V (or Cmd-V on Mac).

The formula that will actually appear in the cells will have appropriate row numbers.

A mouse-based way to do the same is to drag the formula down using the dot in the bottom right corner of the cell.

The easiest thing is to copy and paste; the change from c3:al3 to c4:al4, etc, will happen automatically. I would proceed as follows:

1. Enter the formula =average(c3:al3) in AN3.
2. Select cell AN3, press CTRL+C (or CMD+C on Mac) to copy it (the cell itself, not the formula from the formula line).
3. Select the range AN3:AN562 (or whichever range you want to paste to).
4. Press CTRL+V (or CMD+V on Mac).

The formula that will actually appear in the cells will have appropriate row numbers.

A mouse-based way to do the same is to drag the formula down using the dot in the bottom right corner of the cell.