To calculate conservation of mechanical energy, initial mechanical energy is subtracted from final mechanical energy true or false?

True according to a quick google search.

Yes, if it does not change, then the mechanical energy could be conserved.

However, there is a catch, that catch is that the total mechanical energy could have fluctuated in between. A quantity is conserved if it does not change. The only, definite way to calculate conservation of mechanical energy is to show that its time derivative is 0.

With that being said, the total mechanical energy of a system could have been conserved if the difference between the initial and the final mechanical energy is the 0, but it is not certain.

Think of it like a parabola, if the final and initial mechanical energy are the same, there still can be great variation in between.