How to style a class?

Question

css

Good afternoon, I don’t understand at all, I seem to be doing everything right, I need to set the display: none style on tr_page_top dancing from class 21
, but it doesn’t work, I write in CSS:

.21 #tr_page_top {
display: none;
}

#tr_page_top.21 {
display:none;
}

So and so, in no way, if just #tr_page_top {display:none } then everything is fine

Answer 1

as far as I know the class name cannot start with a number