A75-kg refrigerator is located on the 70th floor of a skyscraper (300 meters above ground). what is the potential energy of the refrigerator?

Here, you can calculate it's potential energy with respect to ground.
We know, U = mgh
Here, m = 75 Kg
g = 9.8 m/s²       [ constant value for earth system ]
h = 300 m

Substitute their values into the expression:
U = 75 × 9.8  × 300
U = 220500 J

In short, Your Final Answer would be 220,500 J 

Hope this helps!

Answermuch larger;

Newtons laws of motion