To stretch a spring 9.00cm from its unstretched length, 19.0 J of work must be done.?

To stretch a spring 9.00cm from its unstretched length, 19.0 J of work must be done.

a) Us = ½kx²
k = 2Us/x²
k = 2(19)/0.09²
k = 4691 N/m
b) F = kx
F = 4691(0.09)
F = 422 N
c) Us = ½kx²
Us = ½4691(0.04²)
Us = 3.75 J
d) F = kx
F = 4691(0.04)
F = 188 N