Tert-butoxide in SN2 reactions
This seems interesting.
The $(CH_3)_3CO^-$ is a strong nucleophile and we observe here methyl bromide is being attacked and there is no point of elimination. There has to substitution in this case.
The bulkier the nucleophile is the more probable is the elimination product due to steric hindrance. In majority of the cases Tert-butoxide undergoes elimination reaction.
The further factor includes temperature at which the reaction is being carried out. 

We can show this relation with $$\delta G  = \delta H - T \delta S$$ 
Hope it was helpful.