3 = k 2^r and 15 = k 2^2r
Multiply first equation by 5 and equate the two equations
NOTE: k 4^2 = k (2^2)^r = k 22r
5 k 2^r = k 2^2r divide out k
5 * 2^r = 2^2r take log of both sides
log 5 + r log 2 = 2r log 2
log 5 = r (2log2 - log2)
log5 / log 2 = r = 2.321928095 (and k = 3/5)