Set it equal to zero. That will allow you to calculate $\displaystyle k_1$. Then, plug in a value to the left of $\displaystyle k_1$ and to the right of $\displaystyle k_1$. You will find the second derivative is zero when $\displaystyle k = 2k_m$. If $\displaystyle k<2k_m$, the second derivative will be negative. If $\displaystyle k>2k_m$, the second derivative will be positive.