We must work out the spring constant first (we will work in cm).

`F = kx`

So `1200 = k(2)`

So `k = 600\ "N/cm"`

So in this case,

`F = 600x`

Now the work done is given by:

`"Work"=int_2^4 600x\ dx`

`=[300x^2]_2^4`

`=3600 text[ N. cm]`

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