I've been reading (trying to brush up on my economics knowledge) and came across the equation: $q_D = a + b(P)$. The variable $a$ is explained as "factors other than price affecting demand."
I read up on non-price determinants of demand (e.g. income changes, preference changes, etc); but, what I've not been able to satisfactorily understand is how $a$ is calculated from those determinants.
So in the equation: $q_D = 6 - 0.5(P)$, what does $6$ represent and how was it determined?