Well, you're on the right path, Benno!
There are some tips regarding VBA programming that might help you out.
Use always explicit references to the sheet you want to interact with. Otherwise, Excel may 'assume' your code applies to the active sheet and eventually you'll see it screws your spreadsheet up.
As lionz mentioned, get in touch with the native methods Excel offers. You might use them on most of your tricks.
Explicitly declare your variables... they'll show the list of methods each object offers in VBA. It might save your time digging on the internet.
Now, let's have a draft code...
Remember this code must be within the Excel Sheet object, as explained by lionz. It only applies to Sheet 2, is up to you to adapt it to both Sheet 2 and Sheet 3 in the way you prefer.
Hope it helps!
Private Sub Worksheet_Change(ByVal Target As Range)
Dim oSheet As Excel.Worksheet
'We only want to do something if the changed cell is B6, right?
If Target.Address = "$B$6" Then
'Checks if it's a number...
If IsNumeric(Target.Value) Then
'Let's avoid values out of your bonds, correct?
If Target.Value > 0 And Target.Value < 51 Then
'Let's assign the worksheet we'll show / hide rows to one variable and then
' use only the reference to the variable itself instead of the sheet name.
' It's safer.
'You can alternatively replace 'sheet 2' by 2 (without quotes) which will represent
' the sheet index within the workbook
Set oSheet = ActiveWorkbook.Sheets("Sheet 2")
'We'll unhide before hide, to ensure we hide the correct ones
oSheet.Range("A7:A56").EntireRow.Hidden = False
oSheet.Range("A" & Target.Value + 7 & ":A56").EntireRow.Hidden = True
End If
End If
End If
End Sub