We can use the Master Theorem to solve this problem.
In this question: a = 3, b = 4, c = 0.75.
So, logba = log43 = log104 / log103 = 0.792.
Because: logba > 0.75, we use the first case in Master Theorem.
(If f(n) = O(nc) where c < logba , then T(n) = theta(n^logba) )
Therefore, T(n) = theta(n^log43).