If FRM_Facilities is the name of the parent (main) form and FRM_Deliverables is the name of the SubForm/SubReport control (not necessarily the name of the subform: the subform and its container control on the parent form can have two different names) where Combo30 is located, your syntax seems correct. However this will give you a pointer (a handle) to the Combo30 control object or to it's default property. These cannot be a valid RowSource property of another ComboBox.
It would be easier to dynamically change the RowSource property of the combo in the second subform (let's call it ComboSecond) from the AfterUpdate Event procedure of Combo30 in the first subform:
Private Sub Combo30_AfterUpdate()
Dim cbo As ComboBox
Set cbo = Me.Parent.FRM_Deliverables.Form.Controls("ComboSecond")
cbo.RowSource = "SELECT ... FROM ... WHERE Deliverables_Type = " & Me.Combo30.Value
' If Deliverables_Type is not numeric use: WHERE Deliverables_Type = '" & Me.Combo30.Value & "'"
Set cbo = Nothing