Please explain how it solve this question...

Dear student
Caesium chloride forms body centred unit cell, hence, body diagonal consists of 1 Cs atom and half chloride atoms on both sides,
also, we know, 
body diagonal=$body diagonal = \sqrt{3}a$
$$\begin{gathered} also, \\ body diagonal = 2(r_{C{s}^{+}} + r_{C{l}^{-}}) \end{gathered}$$
$$\begin{gathered} 1.732\times 350= 2(133+ r_{C{l}^{-}}) \\ \frac{606}{2}= 2(133+ r_{C{l}^{-}}) \\ 303- 133= r_{C{l}^{-}} \\ 170pm= r_{C{l}^{-}} \end{gathered}$$

Regards