Particle dropped on platform with spring - Compression of springs

[rasa]

A massless platform is kept on a light elastic spring.
When a particle of mass 100g is dropped on the platform from a hieght of 24cm it strikes the platform and compresses the spring by 1cm.
From what hieght should the particle be dropped to cause a compression of 4cm?
[text book answer: 396cm]

 

[radou]

Use the fact that energy (kinetic + potential) is conserved.