A Mass of 2kg hung off a spring .which extend 2 cm determine the energy stored in the spring

Answer 1

F = kx ==> k = F/x
E = 1/2 kx^2 = 1/2 * F/x * x^2 = Fx/2 = mgx/2
watch the units

Answer 2

the energy stored in the spring is the change
... in the gravitational P.E. of the mass

E = m g h = 2 * 9.8 * .02 Joules