当需要数组时，我应该如何在视图模型（knockout js）中进行计算？

I am working on a javascript problem with knockout js. I am a developer with a background in C#. Here is my javascript code. I want to use the "concentrations" and also the variable "a" and "b". This is not working. It seems that I can not use "a" for databinding. Once I replace the calculation "var res=..." by "var res = 14" it works. I do not understand this. How can I use my concentrations in a calculation? I need to improve my code. I think it is clear what I want: just do a calculation based on concentrations for variable "a".I am working on a javascript problem with knock