Answer
-6.75 J
Work Step by Step
We found in part (c) that $\Delta U=U_{2}-U_{1}=6.75\,J$
If the gravitational potential energy at the maximum height $U_{2}=0$
Then, $0-U_{1}=6.75\,J$
i.e., gravitational potential energy at the launch point $U_{1}=0-6.75\,J=-6.75\,J$
