How to find role of Constant coeficient DH?

Hello
i have a series of graphs and i know the general form of governing equation is:
p = (a + b*x + c*(x^2)) / (1.0 + d*x + e*(x^2)) + f

(x is Degree)

P is dependent on two things:
1-x
2-a constant parameter DH

it means y changes with both DH and x,
the question is i know the general form of equation which is like above equation,but i must find the role of DH in this equation,for every graph i know the quantity of a,b,c,d,e and f but i do not know what is the relation of this constants with DH,how i can find and insert DH in my equation?is there any method or software that i can find the role of DH in my equation?
for example i need an equation like this:
P=(0.5*DH+(DH^0.2)*x+(DH/2)*(x^2))/(1.0 + d*x+ (DH^0.45*(x^2)) + DH/10

or something like it.

thanks