**HINT** :
Your modification should be $\displaystyle \frac{dx}{dt} = kx-\lambda x^2 - 100$.
Then, you have $\displaystyle \frac{1}{\lambda x^2 - kx + 100}=-dt$.
Can you take it from here?
**HINT** :
Your modification should be $\displaystyle \frac{dx}{dt} = kx-\lambda x^2 - 100$.
Then, you have $\displaystyle \frac{1}{\lambda x^2 - kx + 100}=-dt$.
Can you take it from here?