The Chain Rule & Implicit Differentiation · Hard problem

A firm's output is $Q(K,L) = \sqrt{KL}$ (in thousands of units), where $K$ is capital and $L$ is labor hours. Over time, $K(t) = 100 + 5t$ and $L(t) = 400 - 10t$. The firm wants to know whether output growth is speeding up or slowing down at $t=0$. Find $\dfrac{d^2Q}{dt^2}$ at $t=0$.