With the aid of a scientific calculator (most smart phones are equipped with one), you need to look out for the "LOG" button.
The button usually has the inscription shown as;
[tex]\text{Log}_{10}[/tex]Press the Second Function button and this key changes to;
[tex]\text{Log}_y[/tex]This second function button means,
"The log of x base y".
In this case it means, you take the log of 129, base 6.
The steps are;
[tex]\begin{gathered} (1)\text{Press 129} \\ (2)\text{Press Second Function button} \\ (3)\text{Press 6} \\ (4)\text{Press the "equal to" button} \end{gathered}[/tex]