$\eta = N/(1+2a)$
And $\eta$ is given as $0.5 = 1/2 $
and Given,
$N = 100$
$B = 10Mbps = 10*10^6 bps $
$L = 72000 * 10^3 mtr $
$Propagation Speed = 3*10^8 m/sec$
Means Travel $3*10^8 m ------> 1 sec$
$ 1 m ------> 1/3*10^8 sec$
$72 * 10^6 m ------> 0.24 sec $
Now $Propagation Delay (Tp)= 0.24 sec$
$\eta = 1/2$
$100/1+2a = 1/2$
$a=99.5$
Where ${a=(Propagation time)/(Transmission Time) = Tp* B/L}$
$L = Tp * B / 99.5$
$L=(0.24 * 10*10^6 )/99.5 == 24120b$
divide this by $8$ to get ans in Byte
$L=24120/8=3015 Bytes$
Done!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!