Determine if 1 Queen can take King
I would suggest that rather than try parsing strings all through your code, try this.

The code below creates a couple of new things. a numeric coordinate structure, a scenario, which is a pair of coordinates, and a list of scenarios.

There are also a couple of functions to parse the text file and each chess location into coordinates. Further down the line, it makes thing easier when you come to checking the queen and king locations.

Module Module1
    Structure Coord
        Dim Rank As Integer
        Dim File As Integer
    End Structure

    Structure Scenario
        Dim King As Coord
        Dim Queen As Coord
    End Structure

    Dim Scenarios As New List(Of Scenario)
    Dim fileContents() As String
    &lt;STAThread&gt;
    Sub Main()
        Const PATH = "C:\Temp\kingqueen.txt"
        fileContents = System.IO.File.ReadAllLines(PATH)
        ParseTextFile()
        Dim result As String = ""
        For Each testScenario As Scenario In Scenarios
            result = result &amp; TestLines(testScenario) &amp; " "
        Next
        result.Trim()
        Console.WriteLine(result)
        Console.ReadLine()
    End Sub

    Private Function ParsePieceLocationAsCoord(chessCoord As String) As Coord
        Dim tempPoint As New Coord
        tempPoint.File = Asc(chessCoord(0)) - 96
        tempPoint.Rank = Val(chessCoord(1))
        Return tempPoint
    End Function

    Private Sub ParseTextFile()
        Scenarios.Clear()
        For Each line As String In fileContents
            Dim newScenario As Scenario
            newScenario.King = ParsePieceLocationAsCoord(line.Substring(0, 2))
            newScenario.Queen = ParsePieceLocationAsCoord(line.Substring(3, 2))
            Scenarios.Add(newScenario)
        Next
    End Sub

    Private Function TestLines(testScenario As Scenario) As String
        With testScenario
            If .King.Rank = .Queen.Rank Or .King.File = .Queen.File Then
                Return "Y"
            Else
                Return TestDiagonals(testScenario)
            End If
        End With
    End Function

    Private Function TestDiagonals(testScenario As Scenario) As String
        With testScenario
            If (.King.Rank + .King.File = .Queen.Rank + .Queen.File) Or (.King.Rank - .King.File = .Queen.Rank - .Queen.File) Then
                Return "Y"
            Else
                Return "N"
            End If
        End With
    End Function