VBA Formula

North_Yorks 

If you want to make the same changes on multiple sheets, a more succinct version might be

Option Explicit

Private Sub Workbook_Open()
    HideRows
End Sub

Sub HideRows()
Dim ws As Worksheet
    For Each ws In Sheets
        If ws.Name <> "GeneralInstructions" Then
            ws.Rows("8:18").Hidden = True
            ws.Rows("21:30").Hidden = True
        End If
    Next
    Sheets("GeneralInstructions").Select
End Sub

It is the final (or in this case the only) Select that determines what you see when control is returned from the macro. 

[I didn't bother about EntireRow because the Row object is already the entire row]

Sub UnhideRows()
Dim ws As Worksheet
    For Each ws In Sheets
        ws.Rows.Hidden = False
    Next
End Sub

Hi Craig Hatmaker  Good to see you around!  Do not hesitate to improve my programming style,  It has a long way to go!