Question

An object with the mass 500g was hang from the spring with force constant 20N/m how far in cm would the spring stretch

Answer 1

F = kx
plug in your numbers. watch the units.

Answer 2

weight force = m g = 0.500 * 9.81 Newtons
x = F/k =0.500 * 9.81 N / 20 N/meter
multiply by 100 to get cm