The energy associated with the stone from a height of 7.70 m can be calculated by multiplying the mass, gravitational acceleration and height. We do as follows:
Energy = mgh
Energy at 7.70 m = m(9.81)(7.70) = 75.537m
Energy at 5.25 m = 0.5mv^2 =75.537m
v = 12.29 m/s