The equation for potential energy on a frictionless surface only depends on 2 things, the spring constant and the displacement. In the case of only needing to calculate the PE, the mass of the spring is unnecessary.
where k is the spring constant and x is the displacement from equilibrium.
so for this particular question
=2.0*0.25 N/m *m^2
=0.50 N*m or 0.50 J
a joule (J) is the SI unit of energy and N*m=J, so that is a way you can check you did the math right. keeping track of your unit and doing dimensional analysis.