$\pi_{RS}(r)  \pi_{RS} \left (\pi_{RS} (r) \times s  \pi_{RS,S}(r)\right )$
$\quad= \pi_{a,b}(r)  \pi_{a,b} \left (\pi_{a,b} (r) \times s  \pi_{\color{\red}{a,b,c}}(r) \right)$ [See here I have written a,b,c because $RS$ tuple returning a,b and now adding column of S, here comma operation doing union of columns]
Now, $\pi_{a,b} (r) \times s$ what it returns?
It is doing nothing but concatenation of $a,b$ column of $r$ and $c$ column of $s$
So, it is returning
a 
b 
c 
Arj 
TY 
12 
Arj 
TY 
14

Cell 
TR 
12 
Cell 
TR 
14 
Tom 
TW 
12 
Tom 
TW 
14 
BEN 
TE 
12 
BEN 
TE 
14 
Now, we can do this part of question, which returning nothing but the rows of $r$, which is not in original table
i.e. $ \pi_{RS} \left (\pi_{RS} (r) \times s  \pi_{RS,S}(r)\right )$
And finally,
$\pi_{RS}(r)  \pi_{RS} \left (\pi_{RS} (r) \times s  \pi_{RS,S}(r)\right )$
$\quad=(r/s) $