Explain how in glycolysis that the splitting of one glucose molecule results in 4 atp but there is only a net gain of 2 atp.

In order to make ATP, you have to spend ATP. So Glycolysis need 2 ATP to break down the glucose, which results in 4 ATP, but a net gain of 2 ATP.
For example, if you want to start selling cookies, you first have to put in some money for supplies,right? Lets say supplies are $2. Then you sell your cookies and get $4, but not really, because the $2 was used for supplies

In fact it is not just nuclear fusion and fission that  can  transform  matter  (or mass)into energy. any time  energy  is produced, some small amount of  matter  or mass isconverted into energy.