During 0-1 each task completes its initiate.

During 1-2
   Task 1 completes its request (i.e., the request is granted).
   Task 2's request cannot be granted (not safe).  So 2 is blocked
Three items available at 2.

During 2-3
   Banker first checks blocked tasks but cannot grant 2's request (not safe).
   Task 1 releases 1 unit (available at 3) and terminates at 3.
Four item available at 3.

During 3-4
   Banker first checks blocked tasks and can grant 2's request.
Three items available at 4.

During 4-5
   Task 2 releases 1 unit (available at 5) and terminates at 5.
Four items available at 5.