GDP Deflator

Hello everyone. I’m new here,this is my first post regarding EOC in econ, would much appreciate your input.

GDP deflator values for 2008 and 2010 are 190 and 212.8 respectively, which of the following best describes annual growth rate of overall price level

the solution was (212.8/190)^1/2= 5.8%

so my confusion is the deflator, when it says 2008 =190 and 2010= 212.8 doesn’t that leave out 2009 GDP? why was that left out in the solution?

You are given the formula to compute the geometric mean here, which does not require the in between value.