A spring of force constant 250.0 N/m and unstretched length 0.300 m is stretched by two forces, pulling in opposite directions at opposite ends of the spring, that increase to 18.0 N. What is the displacement of the spring from its unstretched length when subjected to these forces
a) 0.06 m
b) 0.12 m
c) 0.18 m
d) 0.24 m