Form a net population rate with separate growth constants

$dn/dt=kn$ with $k>0$. The derivative represents the rate of change, and “increase” determines its positive sign for a positive population.

Write births per unit time as $ax$ and deaths per unit time as $bx^2$, where $a,b>0$ are separate constants. Then $dx/dt=ax-bx^2$. The teacher explicitly requires the general balance “rate of change = rate of increase − rate of decrease” and warns against using the same letter for unrelated proportionality constants.