Speed Up= Time without Pipeline / Time with Pipeline
=n*tn/[k+(n-1)]tp
where, Time without Pipeline= n*tn
Time with Pipeline= [k+(n-1)]tp
but in the question it is given that "assuming that a very large number of instructions are to be executed" ie. n>>k
therefore ,Time with Pipeline= n*tp (as n is very large so k becomes zero)
now, speedup= n*tn/n*tp
=tn/tp
=12/6 = 2