First, Your last '1' needs to be a 'b', as some others brought up.
Second, I found Goal Seek really doesn't work too well (if you sub the 'b' in), because 1 will always solve this equation. This means that Goal Seek won't really solve it, since there's no way of constraining a minimum on the value, BUT if you want to just automate Goal Seeker, there is this expensive add-in.
Third, The VBA way to run GoalSeek is
where here "A1" would be the cell with the formula in it and "B1" would be the empty cell that would end up with the 'b' value. So you could create a loop for it and automate that way.
Fourth, The best way is to use Solver in VBA. Enable the Solver Add-In, then in VBA enable the Solver reference. Use the following code as an example, where the D column has the formula, the C column is where 'b' will end up, and $H$1 is the value 2, so that we keep it from solving it with b=1:
Public Sub SolveGeometric()
Dim i As Integer
For i = 2 To 3
SolverOk SetCell:="$D$" & i, MaxMinVal:=3, ValueOf:="0", _
ByChange:="$C$" & i
SolverAdd CellRef:="$C$" & i, Relation:=4 'keep it an integer
SolverAdd CellRef:="$C$" & i, Relation:=3, FormulaText:="$H$1"
' $H$1 = 2, so that it won't find 1 which always solves the equation