25 = 100 / (1 + 9e^(-2k))
First I multiplied both sides by (1 + 9e^(-2k)) and got
25 + 225e^(-2k) = 100 + 900e^(-2k)
Then I subtracted 225e^(-2k) from 900e^(-2k) and 100 from 25 and got
-75 = 675e^(-2k)
I divided -75 by 675 and got
-1/9 = e^(-2k)
Now I think I'm supposed to take the natural log (ln) of both sides, but when I take the ln of -1/9 my calculator says "error: nonreal answer" What am I doing wrong?