LINEST, LOGEST, GROWTH or TREND??

Hi all, I need to get the R2 statistic of a formula like this

f(t) = a + b*c^t

I've tried with LINEST(Range Y,LN(Range X),,TRUE) then the array three cells down and it should appear, but the numbers I get applies only to:

f(t) = b*c^t

Anybody knows how to get the real value or the R2 in any other way?

Thanks NlCO