let variable "$a$" has address $100$ and "$b$" has $200$ .
and a variable in which "$a+b$" is stored has address $300$.
now $p(300,100,100)$ which represent $x,y,z$
$y:=y+1$ // it makes $a=3$;
$z:=x+x$ // x means the value contained at address $300$ i.e. $5$
$5+5 =10$ hence value at address $100$ i.e. variable "$a$" will get the value $10$ .
Hence the value of $a$ i.e. $10$ will be printed.