How do I find a backlog item or task from its number?

The easiest way to find a backlog item or task from its number is to use the "Search in all projects" popup, by clicking the search icon in the upper right corner of the page, next to the project selection dropdown:

To search for a backlog item, simply enter the backlog item number. To search for a task, enter "task" followed by the task number, for example "task 4387".