Answer:
a)
, b)
, c)
, d) 
Step-by-step explanation:
a) The spring constant is calculated by using this expression:



b) The work needed to compress the spring from its initial position is:

![\Delta U_{k} = \frac{1}{2}\cdot (300\,\frac{N}{m} )\cdot [(-0.3\,m)^{2}-(0\,m)^{2}]](https://tex.z-dn.net/?f=%5CDelta%20U_%7Bk%7D%20%3D%20%5Cfrac%7B1%7D%7B2%7D%5Ccdot%20%28300%5C%2C%5Cfrac%7BN%7D%7Bm%7D%20%29%5Ccdot%20%5B%28-0.3%5C%2Cm%29%5E%7B2%7D-%280%5C%2Cm%29%5E%7B2%7D%5D)

c) The work needed to stretch the spring is:
![\Delta U_{k} = \frac{1}{2}\cdot (300\,\frac{N}{m} )\cdot [(0.2\,m)^{2}-(0\,m)^{2}]](https://tex.z-dn.net/?f=%5CDelta%20U_%7Bk%7D%20%3D%20%5Cfrac%7B1%7D%7B2%7D%5Ccdot%20%28300%5C%2C%5Cfrac%7BN%7D%7Bm%7D%20%29%5Ccdot%20%5B%280.2%5C%2Cm%29%5E%7B2%7D-%280%5C%2Cm%29%5E%7B2%7D%5D)

d) The work need to stretch the spring is:
![\Delta U_{k} = \frac{1}{2}\cdot (300\,\frac{N}{m} )\cdot [(0.2\,m)^{2}-(0.1\,m)^{2}]](https://tex.z-dn.net/?f=%5CDelta%20U_%7Bk%7D%20%3D%20%5Cfrac%7B1%7D%7B2%7D%5Ccdot%20%28300%5C%2C%5Cfrac%7BN%7D%7Bm%7D%20%29%5Ccdot%20%5B%280.2%5C%2Cm%29%5E%7B2%7D-%280.1%5C%2Cm%29%5E%7B2%7D%5D)

I think each girl had 8.1 points may be wrong
The area of the circle will be 28.27 km.
He'll get $14.03 because 20.25 is what he gave but it only came up to 6.22 so 20.25-6.22=14.03
Distribute.
n(n+3)
Answer : n^2+3n