law of mass action

INeedHelpTY

1. The problem statement, all variables and given/known data
Given the SIR model:

dS/dt = a(S+I+R) - (bI+a)s
dI/dt = bIS - (a+c)I
dR/dt = cI - aR

S(0) = S₀
I(0) = I₀
R(0) = R₀

Show that the equations can be derived from the law of mass action, in other words, find the reactions (reactants --> products) that would give rise to the equations above.

I wanted to know if you guys know any site, any text that i can consult with, to solve the problem. Thanks.