$\begin{array}{*{20}{c}}{a = {2^5} \Rightarrow \log _2^a = 5}&{b = {2^7} \Rightarrow \log _2^b = 7}\end{array}$$\log _8^{ab} = \log _{{2^3}}^{ab} = \frac{1}{3}\log _2^{ab} = $$\frac{1}{3}\left( {\log _2^a + \log _2^b} \right) = \frac{1}{3}\left( {5 + 7} \right) = \frac{1}{3} \times 12 = 4$