in my previous question why are we using the formuka f=v x d x g

This formula is according to Archimeds' law of buoyancy.

According to this law the buoyant force on an object fully or partially immersed in a fluid is equal to the weight of fluid displaced by that object.

So in the question I have taken air as a fluid which the baloon displaces as it is immersed in the fluid.

Thus weight of air displaced = (voulme of air displaced) x (density of air) x (gravitational acceleration)