For a reaction that proceeds by E1 mechanism, the rate determining step involves the formation of the carbocation.
The rate of formation of this carbocation depends only on the concentration of the t-butyl bromide since it is the only specie that enters into the rate equation.
Hence, when the concentration of t-butyl bromide is tripled, the rate of reaction is tripled.
Methanol does not enter into the rate equation hence doubling its concentration does not affect the rate of reaction.
