Answer:
You don't have a question but I'll answer what I can.
half-life = natural log (.5) / k
half-life = -.693147 / .1247
half-life = 5.561106656
It seems you did not express the k-value properly.
1) A k-value should have a negative value.
2) A k-value should also have units.
For example, the k-value of tungsten 181 is
-0.005723757 / days
Explanation: