bandwidth of channel is given as 75 Tbps means in 1 second 75 Tb data can be transmitted
means $\frac{75\ TB}{8}$ data can be transmitted in 1 second .
$1\ B \rightarrow \frac{8\ sec}{75*10^{12}}$.
there are given 64 bits for sequence bits. ,so we can use $2^{64}$ sequence no uniquely.
for $2^{64}$ sequence no. we can use $2^{64}$ bytes data.
$2^{64}\ B \rightarrow \frac{8\ }{75*10^{12}}*2^{64} seconds$.=1967652 seconds
wraparound time =1967652 seconds=546.57 hours=22.7 days=3.2 weeks =around 3 weeks.