Pure (R)-2-bromo-1-fluorobutane reacts with sodium cyanide in aqueous ethanol to form pure (R)-2-(fluoromethyl)butanenitrile.

CH3CH2CHCNCH2F2-(fluoromethyl)butanenitrile

a. What is the probable mechanism of this reaction? Explain your reasoning.

I think the mechainsm is Sn2

b. Based on your choice of mechanism, explain how both the substrate and the product can have the R configuration.
pure substrate

Please help me, I am really having hard time with this question