You will need to define the variables first and save them as bands (Lambda_B10, c2, LSE). Afterwards you write your first formula using the saved variables (they will appear in your band list). Which LSE will you use? (I understand is emissivity ?).

Yes emissivity. I wanted to know if the syntax of the formula as it is the correct for what the Band Math of SNAP requires as necessary to be written as expression?

Besides, is it possible to be calculated/obtained Land and Sea Surface Temperature from Sentinel 3 band/s with a smaller pixel size than 1 Km?

If exists, what tutorial can I use to follow up explanations on which specific Sentinel 3 product to download and how to process it to obtain LST in such higher spatial resolutions?