Let [tex] d [/tex] be the number of slices eaten by Davonne, and [tex] r [/tex] be the number of slices eaten by the runner-up.
The sentece "Davonne ate two more than twice as many slices as the runner-up" translates as
[tex] d = 2r+2 [/tex]
In fact, [tex] 2r [/tex] is twice as much as the runner-up ate, and the final "+2" represents the "two more than.." part.
Since we know hoy many slices Davonne ate, let's plug that value and solve for [tex] r [/tex]:
[tex] 11 = 2r+2 \iff 9 = 2r \iff r = \cfrac{9}{2} = 4.5 [/tex]
So, the runner-up ate 4.5 slices.