SYN flag used to create sequence number of a connection.
Here client has sequence number $100$ and server has sequence number $500$
So,
- client send SYN flag of $100$ to server
Server sends and SYN=$500$ to client. Server also wants to know next byte of client ,So, sends ACK=$101$
Next client sends ACK $501$
- After that, two $100B$ packet sends by client.
So, after first $100B$ packet server sends ACK $201$ and after next $100B$ packet server sends ACK $301$
So, at final stage client has ACK=$501$ and server has ACK=$301$