Well I have some other approach but answer as 3500Joules see how.......

Suppose at the height of 15m the body was left free....but as it has some velocity i.e. 20m/s at that instant...then it will continue its upward motion for some distance and attain a maximum height....

and thus Potential Energy of the Body at that instant will be the Total Work Done on the Body.....

so.....using

we have....

using g = 10.......gives h = 20 m...

so final height = 15 + 20 = 35 m.......

and Potential Energy at 35m from ground = mgh = 3500 Joules........

If somebody has some other approach then pls do post.........