Hi shreya.. :)
Had it been x2A then, you can follow the usual method of equating MRS of both individuals to get Pareto Efficient Points.
You will get 45 degree line in the edgeworth box.
However, if its given x2B in the book, then it becomes a problem of externality.
In order to find Pareto Efficient Points in this case, you can substitute (10-x2A) in case of x2B
[since, total demand for good2 i.e x2A + x2B= total endownment of good 2 i.e 10 units ]
and then proceed in the same way as above.
:)