Hello Raja

Here:

ΔTb=0.45K

w=0.4g weight of Anthracene.

W=25g weight of chloroform

Kb= Molal elevation constant(3.9K−Kg/mol)

m=Kb×w×1000W×ΔTb=3.9×0.4×25000×0.45          =17550kg/mol

I hope you understand my explanation and got cleared with the point. You can ask if you have any doubts in the comments section.

ALL THE BEST