Answer
The reaction rate triples.
Work Step by Step
The given reaction is $S_N1$ reaction. It is of the first order overall. It is first order with respect to the substrate and zero order with respect to the nucleophile.
Rate = $k[C_4H_9Br]$
So, the reaction rate triples if the concentration of potassium tert-butoxide is tripled and the concentration of ethyl bromide is doubled