$
\begin{array}{l}
a. \\
2^x = 6 \\
x = {}^2\log (6) \\
\\
b. \\
7^{2x + 5} = 2^3 \\
7^{2x + 5} = 8 \\
2x + 5 = {}^7\log (8) \\
2x = {}^7\log (8) - 5 \\
x = \frac{1}{2} \cdot {}^7\log (8) - 2\frac{1}{2} \\
\\
c. \\
11^{x^2 - 4x} = 121 \\
11^{x^2 - 4x} = 11^2 \\
x^2 - 4x = 2 \\
(x - 2)^2 - 4 = 2 \\
(x - 2)^2 = 6 \\
x - 2 = \pm \sqrt 6 \\
x = 2 \pm \sqrt 6 \\
\end{array}
$