I'm new to Mathematica and while it seems like there should be a way to do this (it is effectively simultaneous equations) I don't know how to input it to Mathematica. I've tried using solve and setting it in a table first but it seems I'm not realizing something or missing something in the documentation about how to play with a few.
The problem represents light passing through tissue. There are two variables in the equation that I need to know, a and b. (if there is a method to input equations/code here can someone point it out to me?)

I(lam), I0(lam) are measured values. I will know these, they are the intensity of the light coming out and going into the volume at wavelength lambda. M(lam) is the absorption of tissue at the wavelength lamda, this I will also know at all lambdas.
d thickness of the tissue, this is a set value.
a and b are what I would like to know. That term represents the light scattering function for the tissue.
What I would like mathematica to do is tell me how many lambda's, or colors of light, do I have to measure at to find what a and b are, and the equation that gives me a and b.