gravitational Potential energy is given by
[tex]GPE = mgh[/tex]
here we have
m = 200 kg
h = 21000 m
now we will have
[tex]GPE = 200(9.8)(21000)[/tex]
[tex]GPE = 4.116 \times 10^7 J[/tex]
so GPE of balloon will be 41160000 J above the given height from ground