How does one complete a summing operation where

$\displaystyle a = 11$ and $\displaystyle b= 96$

$\displaystyle X= {8, 10, 9} $ and $\displaystyle Y= {108, 101,103,94}$

a sum like

$\displaystyle i=1$ and $\displaystyle n=4$ for $\displaystyle sigma(Yi-a-b)Xi$

where i is the index

i expanded it like this:

$\displaystyle (Y1-a-b)*X1 + (Y2-a-b)*X2 .... (Yn-a-b)*Xn$ and in this case n = 4

My question, assuming I expanded correctly, stands thus: since the set of X has only 3 elements so how do I compele this operation?

thanks!