$0/25 = \frac{{25}}{{100}} = \frac{1}{4} = {4^{ - 1}} \Rightarrow \log _4^{0/25} = \log _4^{{4^{ - 1}}} =  - 1 \Rightarrow a =  - 1$ $\begin{array}{l}128 = {2^7} \Rightarrow \log _2^{128} = \log _2^{{2^7}} = 7 \Rightarrow b = 7\\ \Rightarrow a + b =  - 1 + 7 = 6\end{array}$