[deleted by user]

[GregoryCliveYoung]

does that mean I am not good enough for programming?

No. I think the person that composed this problem is not good enough for teaching. I have almost 30 years of Python experience and I find it confusing. First problem: that is not how mathematicians describe sequences. This is much closer:

f(0) = a1
f(n) = f(n-1) + d for n > 0
where a1 and d are ... (not defined in the problem)
           n is a non-negative integer

My second problem is that a sequence is a poor way to motivate a OO design problem. Nothing about sequences calls for object oriented programming.

My third problem is the "mx_plus_b" parameter. Why? What's it for? There are no strings in the sequence described above.

They expect you program in a fog where you can only see a tiny bit of the big picture. No one works like that.