The server's reply carries its own starting sequence number and acknowledges the client's SYN. The acknowledgment is recomputed as one past the client's sequence number.
highlighted = computed this step
Why the reply carries two facts
The server's SYN-ACK both publishes the server's starting number and acknowledges the client's SYN. The acknowledgment is recomputed from the earlier client sequence number.
server ISN=3000
The first acknowledgment
The client sent sequence number 1000. Because SYN consumes 1 sequence number, the server acknowledges 1001.
1000+1=1001
The server chooses its own start
The same segment also carries the server sequence number 3000, so the client can acknowledge the server next.
server seq=3000
Summary
The second segment proves the client SYN was seen and exposes the server's starting sequence number. Connection setup ordering and exact sequence numbers only; timing, retransmission, and round-trip time are not modeled here.