Hi

I don't know what the meaning of "special" in "special arithmetic progression is...

Let r be the constant progression of this sequence.

We have $\displaystyle 731=663+2r$ because they're separated by 2 terms.

So $\displaystyle r=34$

Now, note that $\displaystyle 731=34*21+17$

It all depends on where you want to start the sequence, but if it has to be the first positive term, it would be $\displaystyle a_0=17$ and $\displaystyle M=34$

Shouldn't it be M = 21 ?
With these informations, I guess you can continue, can't you ?