The two users do not interact in this use case and should both be on the left (neither are an addition system that is invoked).

You could say that the two users have slightly different use cases - clients can only cancel their requests, travel agents can cancel any request. You can then say they both extend the same basic use case.

See approximate diagram bellow (their should only be one use case "Cancel a request" at the right with two arrows entering it).

Also, don't forget to box the system's boundaries.

                   ---------------------------------------------------------------
                   |                                                             |
    Client --------|--> (Cancel own request)--<<includes>>-->(Cancel a request)  |
                   |                                                             |    
    Travel agent --|--> (Cancel any request)--<<includes>>-->(Cancel a request)  |
                   |                                                             |
                   ---------------------------------------------------------------