When the concentration of alkyl halide is tripled and concentration...

Rate of ${S}_{N}2$ reaction is as follows:
Rate = [alkyl halide] $\times$ [nucleophile]
Rate (initially) $= \left[{C}_{1}\right] \left[{C}_{2}\right]<br />At new concentration<br />$ Rate \left(finally\right) =\left[3{C}_{1}\right]$\left(\dfrac{{C}_{2}}{2}\right) =1.5 \times \left(\left[{C}_{1}\right]\left[{C}_{2}\right]\right)$
\text{Rate} \left(\text{final}\right)\text{=}\text{1}\text{.5}\text{\times}\text{Rate}\left(\text{initial}\right)