<PAGE>
 
                            MALCOLM I. GLAZER TRUST
                            c/o Zapata Corporation
                        1717 St. James Place, Suite 550
                             Houston, Texas 77056

                                             September 20, 1995

Enterra Corporation
13100 Northwest Freeway
Houston, Texas 77040

Dear Sirs:

  Reference is made to the agreement dated as of this date among Zapata 
Corporation ("Zapata"), Energy Industries, Inc. ("Zapata Sub"), Zapata Energy 
Industries, L.P. ("Zapata Partnership"), Enterra Corporation ("Enterra") and a 
subsidiary of Enterra ("Enterra Sub"), pursuant to which Enterra Sub will be 
acquiring all of the assets of the natural gas compression businesses conducted 
by Zapata Sub and Zapata Partnership (the "Sale Agreement").

  To induce Enterra and Enterra Sub to enter into the Sale Agreement, the 
Malcolm I. Glazer Trust (the "Trust") hereby agrees, on behalf of itself and any
affiliates of the Trust or Malcolm I. Glazer who own shares of common stock of 
Zapata ("Affiliates"), that the Trust and all Affiliates will vote all shares of
common stock of Zapata owned or controlled by them in accordance with the 
recommendation of the Board of Directors of Zapata with respect to the approval 
by the stockholders of Zapata