**Hint:** The derivative of $e^{-KX}$ is $-KX e^{-KX}$. Set your $f'(X)=0$, re-arrange your equations and then take logs of both sides of the equation to solve for $X$.
**Hint:** The derivative of $e^{-KX}$ is $-KX e^{-KX}$. Set your $f'(X)=0$, re-arrange your equations and then take logs of both sides of the equation to solve for $X$.