Economy Q:

[Guest51]

If an indifference curve is totally flat and Y is a typical good with finite
marginal utility, what is the marginal utility of X?

[carrot99]

Marginal utility = total utility difference / quantity of goods difference

Find the total utility of the first event.
Find the total utility of the second event.
Find the difference between both (or all) events.
Find the difference between the number of goods between both (or all) events.
Apply the formula.

The site where I found this gives an example for you to follow towards the bottom of the page.

https://www.indeed.com/career-advice/career-development/how-to-calculate-marginal-utility