Do you mean dh/dt? In this problem we were solving for dx/dt, so it was unknown. In, general, you can only ever solve for one variable in one equation. So, you would either need an additional piece of information, or be provided with additional information so you can set up a system of equations.
dh, in this case means a small change in h. dt refers to a small change in time. Thus the fraction, dh/dt, refers to the rate of change of h, with respect to time.
