PE = mgh where m is the mass in kg, g is the pull of gravity which is 9.8, and h is the height of the object above the point to which it could possibly fall, measured in meters. Plugging in:
PE = 10(9.8)(20) so
PE = 1960 J
This should be rounded to 1 sig fig according to the rules of sig fig and your numbers here, but I imagine you're not following them all that much. It should be 2000 J
