I found delta L to be .002 m as well.
Here's what I did.
.5 N=k*.01 m
m*w^2=k*dL - m*w^2*dL
When you plug in the values (m=.1 kg; w=1 rps; k=50 N/m) the answer is dL=.1/49.9
So dL = .020 m