Insert Relevant Value Based On Selection

I have a form with a combo box this combo box looks up values in my STC table. The STC table has two fields STC & Resolver and each STC has a unique associated Resolver. What i want to happen is after the user has selected the STC from the combo box on the form the associated Resolver be displayed in a textbox on the form. I am ussing the following code but nothing seems to be happening:

Private Sub cboSTC_AfterUpdate()
Dim ResolverSource As String

ResolverSource = "SELECT tblSTC.[Resolver] " & _
"FROM tblSTC " & _
"WHERE tblSTC.[STC]='" & Me.cboSTC.Value & "';"

Me.txtResolver.RowSource = AgentSource
Me.txtResolver.Requery


End Sub