The above substitution mechanism is bimolecular (SN2) because there is a strong nucleophile given as well as an aprotic solvent. Remember: a strong nucleophile favors SN2 and an aprotic solvent will also favor SN2.
The above substitution mechanism is unimolecular (SN1) because there is not a strong nucleophile present. Remember: a weak nucleophile favors SN1 and because the electrophile leaves first, a carbocation is formed. Therefore, the nucleophile, CH3CH2CH2OH can attack both the front and the back, resulting in a racemic mixture.