Question 892783
Stick with the lower-case, k.  This will become a negative value according to the form for the decay model you seem to want.  You are expecting to use {{{highlight_green(y=Yo*e^(kt))}}}.


You want to find k, knowing Yo, t, and y.
Just solve your equation in general, outlined in green shown earlier, for k.  THEN substitute your values for Yo, t, and y.


{{{ln(y)=ln(Yo*e^(kt))}}}
{{{ln(Yo)+ln(e^(kt))=ln(y)}}}
{{{ln(e^(kt))+ln(Yo)=ln(y)}}}
{{{kt*ln(e)+ln(Yo)=ln(y)}}}
{{{kt*1=ln(y)-ln(Yo)}}}
{{{highlight(highlight(k=(1/t)ln(y/Yo)))}}}