Randomize Timer
For i =  1  To  5 
For j =  1  To  3 
A(i, j) = Int(Rnd *  20 )
Text1.Text = Text1.Text & A(i, j) & " "
Next j
Next i
Randomize Timer
For i =  1  To  5 
For j =  4  To  6 
A(i, j) = Int(Rnd *  20  -  20 )
Text1.Text = Text1.Text & A(i, j) & " "
Next j
Next i

Public Function Random(ByVal Lowerbound As Integer, ByVal Upperbound As Integer) As Integer
    Randomize
    Random = Int((Upperbound - Lowerbound) * Rnd + Lowerbound)
End Function

Dim RandomNumber as Integer

...
'в первом цикле
RandomNumber = Me.Random( 0 , 20 )


...
'во втром цикле
RandomNumber = Me.Random(- 20 , 0 )

Private Sub Command1_Click()
Dim i As Byte
Dim j As Byte
Dim A( 4 ,  5 ) As Integer
Dim s As String

Me.Text1.Text = ""
For i =  0  To  4 
    s = ""
    For j =  0  To  5 
        If j <  3  Then
            A(i, j) = Me.Random( 0 ,  20 )
        Else
            A(i, j) = Me.Random(- 20 ,  0 )
        End If
        s = s & A(i, j) & vbTab
    Next j
    Me.Text1.Text = Me.Text1.Text & s & vbCrLf
Next i


End Sub

Public Function Random(ByVal Lowerbound As Integer, ByVal Upperbound As Integer) As Integer
    Randomize
    Random = Int((Upperbound - Lowerbound) * Rnd + Lowerbound)
End Function