@Carl Love ah that delta(k) on S(k+1) is actually a constant. It is a result of differential transformation method.

And delta on I(k+1) is also a constant.

yeah i mean if there's no sum like in eq. I and eq. R there's no problem for me, but i'm stuck at eq. S and eq. E

my bad for wrong choice of words

@Carl Love actually i have this system that needed to be plotted. 

but since i can do the I(k+1) and R(k+1), i still have problems with S(k+1) and E(k+1)..

@Carl Love thankyou so much! it's working now