Question

Write a expression for gravitational potential energy of an object with mass x placed at height y

Answer 1

gravitational potential energy = mass* acceleration due to gravity (g) * height 
So for your case, gravitational potential energy= x*g*y (g=9.8 m/s^2)

Answer 2

the expression for gravitational potential energy of an abject with-
mass=x
heigt=y
 is x *y*g