Suppose a does not equal 0. Compute log_{4^a} 8^b in terms of a and b.
log 4^a 8^b
Using the change of base theorem
x = log 8^b / log 4^a
x = b log 8 / ( a log 4)
x = (b/a) * log 8 / log 4
x = (b /a) * (3/2)
x = (3b) / (2a)