If
[tex]\dfrac{dh}{dt} = -\sqrt{5h} = -\sqrt{5} \cdot h^{\frac12}[/tex]
then its derivative, using the power rule, is
[tex]\dfrac{d^2h}{dt^2} = -\sqrt{5} \cdot \dfrac12 h^{1-\frac12} = -\dfrac{\sqrt5}2 h^{-\frac12} = -\dfrac{\sqrt5}{2\sqrt h} = -\sqrt{\dfrac{5}{4h}}[/tex]
At h = 16, the second derivative has a value of
[tex]-\sqrt{\dfrac{5}{4\cdot16}} = -\sqrt{\dfrac{5}{64}} = -\dfrac{\sqrt5}{8} \approx -0.279508[/tex]
Your answer is essentially correct. Either what you submitted doesn't have enough precision or you're expected to give an exact answer (-√5/8).
