You have correctly worked out that as the distance from the centre. To where the seamen push is 2m and each seaman pushes with 300N each seaman will give a momment of 300*2=600Nm this is then multiplied by four to give a total momment of 4*600=2400Nm.
Then the new part is that to convert from the momment at the end of the projections to. That of the rope setup an equation like this:
2400 = .5 * x
This is as whatever the momment is at the capstan will be half a meter from the centre and four times due to there being foursemen.
X is found to be 4800 N as required