Answer
$ t = 4.28 \times 10^9 y$
Work Step by Step
$ -\lambda t = ln [\frac{N_K}{N_K + N_{Ar} + N_{Ca}}] $
$-\frac{ln2}{T_{1/2}} t = ln [\frac{N_K}{N_K + N_{Ar} + N_{Ca}}] $
$ t = - ln [\frac{N_K}{N_K + N_{Ar} + N_{Ca}}] \frac{T_{1/2}}{ln2}$
$ t = - ln [\frac{1}{1 + 1 + 8.54}] \frac{1.26 \times 10^9 y }{0.693}$
$ t = 4.28 \times 10^9 y$
