Your acetylide anion is your nucleophile.
A proton has been removed from the acetylene, giving a lone pair and a negative charge. This lone pair will attack the carbon on the bromomethane substrate, displacing the bromide leaving group.
The questions specifies that it is an Sn2 reaction, so the attack happens at the same time as the bromide leaves. The displacement occurs with inversion of the stereochemistry at the carbon - but as the other groups attached to the carbon are all the same (i.e. protons) there isn't much point worrying about stereochemistry.
Your products are CH3CCCH3 and bromide. The question doesn't specify the counterion for the acetylide, so you can't specify the counterion to the bromide in the product.
