here we have used sliding window protocol.....
tx=L/B .....that is 32*8/(128*10^3)= 2 mili second....(all are in BITS )
now they given in question that signal travels 2*10^8 meters in 1 sec hence to travel the distance of 10^6 how much time we need=
Tp=10^6/(2*10^8) sec that is = 5 mili second...
now for SLIDING WINDOW PROTOCOL we have formula as 1+2a......(a=Tp/Tx)
by putting all values .....1+2*(5)/2 .....(mili /mili cancel)......by solving that you will get it as 6......