I might be wrong but this is my attempt:
There are two extensions taking place here - firstly from natural length 15x10^-2m to 17x10^-2m - ΔL=2x10^-2
- second from the first extension 17x10^-2m to the new extension 20x10^-2m - ΔL=3x10^-2, however we want the extension from natural length which is 5x10^-2
The question asks for the extra work done so I think you need to work out the difference between the two stretches from natural length, which would be as follows:
Stretch 1: F=kΔL=300x2x10^-2=6N
Stretch 2: F=kΔL=300x5x10^-2=15N
Then you know elastic energy stored is given by E = 1/2xFxΔL
So ΔE = (1/2x15x5x10^-2)-(1/2x6x2x10^-2)
= 0.315J
Like I said, this is probably wrong so don't take my word for it especially as I haven't yet covered this topic in lessons (and exams are in 5 weeks - not great!)