First, it’s always a good idea to add a link to your site, so we can examine it to see if your CSS is correct. For example, is that first right brace } from a previous CSS rule, or did you really add that as part of the new rule?
Second, are you trying to target all page titles, or just certain pages? It looks like you’re trying to hide the title on all pages.
Third, why do you think the page titles have a class of entry-title? While that may be true for other themes, if you inspect the page title on a site using the Corpo theme using a tool like Firebug or Chrome Developer Tools, you’ll see that the page title is an h2 element without class or ID assigned to it (oddly enough). But it is the first child element of a section element with an ID of main-content, so to hide the page title on all pages, add this CSS rule:
#main-content > h2 {
display: none;
}