Correct answer is : 17190
\(\lambda t = ln \cfrac {(^{14}C/^{12}C)_{atmosphere}}{(^{14}C/^{12}C)_{wood\,sample}}\)
As per the question,
\(\cfrac {(^{14}C/^{12}C)_{wood}}{(^{14}C/^{12}C)_{atmosphere}} = \frac {1}{8}\)
So, \(\lambda t = ln 8\)
\(\frac {In \, 2}{t _ {1/2}}t= ln 8\)
\(t = 3 \times t _ {1/2} = 17190\,years\)