Plotting second order reaction with different reactants


> 
> In a laser flash photolysis experiment, $\ce{C6H5NH2}$ $(C\_0 = \pu{60 \mu M})$ is oxidize to its corresponding radical cations. There are three possible reaction pathways for the formation of the cations:
> 
> 
> 1. Dimerization of the radical cations;
> 2. Reaction between the cations and the initial substance; and
> 3. Reaction between the cations and the solvent.
> 
> 
> Determine with the help of the experimental data which is the most
> likely pathway and the rate constant of the reaction.
> $$
> \begin{array}{l|cccccccc}
> \hline
> t \text{ after the pulse (in µs)} & 0 & 5 & 10 & 15 & 20 & 25 & 30 & 35 \\
> \hline
> \text{[Radical cation] (in µM)} & 20 & 11.12 & 6.86 & 4.47 & 3 & 2.06 &1.43 &1 \\
> \hline
> \end{array}
> $$
> 
> 
> 


I understand how to plot for (1) and for (3), however how do I plot for the second pathway? I know that I will have the reaction type:


$$\ce{ A + B -> P}$$


and then I should plot $\ln\left(\frac{a-x}{b-x}\right)$ against $t$ however, how do I actually calculate $(a-x)$ and $(b-x)$?


