how to select a range and then add a specific number of cells after it

Hello all,
I would like to select a range and add a specific number of cells to it. This is what I have so far:

 
    Dim LookBackPeriod As String
    LookBackPeriod = Range("i21").Value
       
       Range("a3:b" & LookBackPeriod & "").Select

i want to do something like ("a3:b2+2") for a range but am unsure how to write the +2 part. Any help is greatly appretiated.
Thanks,
dustin