Gravitational potential energy is given by
U=mgh
where
m is the mass of the object
h is the height of the object above the ground (Earth's surface)
g is the gravitational acceleration constant, 9.8ms2.
For h=6.15m and U=4042J, we can rearrange to solve for m:
m=U/gh
m=(4042J)/((9.8m/s^2)*(6.15m))
m=67.06 Kg