A flower pot 3000 g is sitting on a windowsill 35 meters from the ground . How many joules is this?

Potential Energy is energy stored in the system and ready (potentially) to be changed into, say, Kinetic Energy through movement.
The general expression for Gravitational Potential Energy is:
U
=
m
g
h
where:
m
g
is the weight;
h
is the height relative to a certain reference level (the ground in this case) choosen to have zero Potential Energy.
With your data we get:
U
=
3
⋅
30
=
90
J