Clear[g,h,k,m]

Solve[((-1)/2) k (s^2) + m g (h+s)==0,s]

g=9.8;
h=0.8;
k=1600.;
m=1.20;

sol=Solve[((-1)/2) k (s^2) + m g (h+s)==0,s]

sol[[1,1,2]]

deltaH=sol[[1,1,2]]-sol[[2,1,2]]

deltaH/2

eQuilibrium=((sol[[1,1,2]]+sol[[2,1,2]])/2)