(1)
Also, income I depends on selling price s (rupees/unit)
Thus
I = sx (2)
The profit P is then the difference between income and costs So
P = I – C
= sx – (a + bx)
= (s – b) x – a (3)
We now have a mathematical model of the relationships (1)  to  (3) between
the variables x, C, I, P, a, b, s