Answer
The reaction rate becomes 6 times if the concentration of potassium tert-butoxide is tripled and the concentration of ethyl bromide is doubled
Work Step by Step
The given reaction is $S_N2$ reaction. It is of the second order overall. It is first order with respect to both its reactants.
Rate = $k[EtBr][KO-t-Bu]$
So, the reaction rate becomes 6 times if the concentration of potassium tert-butoxide is tripled and the concentration of ethyl bromide is doubled