physics
posted by Rachel .
A system consists of three particles, each of mass 2.10 g, located at the corners of an equilateral triangle with sides of 22.0 cm.
Calculate the potential energy of the system.
Please help me what formula to use please

Relative to a situation where the masses are widely separated, the potential energy of the equilateral configuration will be negative. That is because it will require work to separate them.
A single mass m has no potential associated with it.
When you add a second mass m at distance a, there will potential anergy equal to
G m^2/a
When you bring in a third mass m at distance a from both, you add potential energy equal to 2G m^2/a
The total potential energy is therefore
3G m^2/a
G is the universal constant of gravity, m = 0.0021 kg and a = 0.22 m
Expect a very small number. 
The constant G can be found at
http://en.wikipedia.org/wiki/Gravitational_constant 
Thank you very much sir!