cascading combo box question

[elvin_08]

hi to all
im new to MS access
i have a table COMPLAB1_PC, COMPLAB2_PC, SPEECHLAB_PC, TECHLAB_PC and BORROWING

in the BORROWING form i have 2 cascading combo boxes SelectLaboratory and ComputerName that lookup for the ComputerNumber of the selected laboratory

then i have this code in the second combo box to change the availability of the selected PC
If (__________) Then
CurrentDb.Execute ("UPDATE SPEECHLAB_PC SET AVAILABILITY = 'NOT AVAILABLE' WHERE [ComputerName] = '" & Me.ComputerName & "';")

i dont know what would i inserted in the red line..
TIA
God Bless