Dear candidate... hope you're doing good..

Well, the tension T in the string attached to the 2kg vertical mass is equal to the downward force which is the weight of the object. So, mathematicaly, we have

T=W=mg=2*9.8=19.6N.

So the net force in the string is

=F-T which is the required answer.

Hope this helps you out....

Best of luck:)