Marginal utility is defined as the additional satisfaction that a consumers gets when he/she consumes an additional unit of a certain good or service.
Marginal utility can be calculated by subtracting the utils of consuming the additional (2nd) unit from the utils of consuming the 1st unit.
Applying this to the question here:
Marginal utility = utils of 2nd banana - utils of 1st banana
Marginal utility = 490 - 350 = 140
