Removing Header, Footer & Main Menu from Single Page

Hello, I have tried all of this code:

.page-id-9788 .site-header {
display: none;
}
.page-id-9788 .site-footer {
display: none;
}
.page-id-9788 #masthead, .page-id-X #sidebar-footer {
display: none;
}
.page-id-9788 .header-image {
display: none;
}
.page-id-9788 .top_bar {
display: none;
}

.page-id-9788 .footer_bar {
display: none;
}

Nothing has worked. Can you help? It’s a landing page and I want it to be full.
Thank you.

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