Hi Niket! First of all you need to draw the chair conformation for this question. You'll find that both of the groups are in equatorial position which is already the most stable form. But, in order to calculate the energy required for chair to flip you need to add the energy absorbed by each substituent to go on axial position. For ethyl value is 8 Kj/mol and for methyl we need 7.6 Kj/mol that give us a total 15.6 Kj/mol of energy required for chair to put substituents on axial positions. Conversely, the same amount of energy will be released for the second chair flip when both substituents will come to be equatorial again.
Further you can find these values and complete detail about how to calculate the strain energy for a stable conformation in this video below: