Differential equations — practice question

In a particular chemical reaction, the quantity $x$ grams of a substance is falling. The differential equation connecting $x$ and $t$, where $t$ is the time in seconds since the reaction began, is $\frac{dx}{dt} = -k x \sqrt{t}$, where $k$ is a positive constant. It is stated that $x = 100$ at the beginning of the reaction.