The easiest method would be to do this in the code behind. Either at page_load or triggered some other way.
I think your question is answered here: Windows Phone 8 - reading and writing in an existing txt file in the project
Using this method, I'd store your flash cards in the Assets folder and read them from there.
For your case, you would want to name your Textblock and then set the text from your code behind.
XAML:
<Textblock x:Name="tbFlashCard" Height="150" TextWrapping="Wrap" Text="**how do i link this**">
Modified code from other post:
tbFlashCard.Text = FileHelper.ReadFile(@"Assets\MyTextFile.txt");