I assume that we have this ??
y = a(b)^x
y = a(8)^x divide both sides by a
(y / a) = (8)^x take the log of both sides
log( y /a ) = log 8^x and we can write
log ( y / a) = x * log 8 divide both sides by log 8
log ( y / a) / log 8 = x swap x and y
log ( x / a) / log 8 = y and for y, write f-1 ( x)
f-1(x) = log ( x / a) / log 8